Add improved docs

Author: wooorm

readme.md

@@ -8,46 +8,94 @@
 [![Backers][backers-badge]][collective]
 [![Chat][chat-badge]][chat]
 
-Extension for [`mdast-util-from-markdown`][from-markdown] and/or
-[`mdast-util-to-markdown`][to-markdown] to support GitHub flavored markdown
-(GFM) footnotes in **[mdast][]**.
-When parsing (`from-markdown`), must be combined with
-[`micromark-extension-gfm-footnote`][extension].
-
-GFM footnotes were [announced September 30, 2021][post] but are neither
-specified nor supported in all their products (e.g., Gists).
-Their implementation on github.com is currently quite buggy.
-The bugs have been reported on
-[`cmark-gfm`](https://github.com/github/cmark-gfm).
-This micromark extension matches github.com except for its bugs.
+[mdast][] extensions to parse and serialize [GFM][] footnotes.
+
+## Contents
+
+*   [What is this?](#what-is-this)
+*   [When to use this](#when-to-use-this)
+*   [Install](#install)
+*   [Use](#use)
+*   [API](#api)
+    *   [`gfmFootnoteFromMarkdown()`](#gfmfootnotefrommarkdown)
+    *   [`gfmFootnoteToMarkdown()`](#gfmfootnotetomarkdown)
+*   [Syntax tree](#syntax-tree)
+    *   [Nodes](#nodes)
+    *   [Content model](#content-model)
+*   [Types](#types)
+*   [Compatibility](#compatibility)
+*   [Related](#related)
+*   [Contribute](#contribute)
+*   [License](#license)
+
+## What is this?
+
+This package contains extensions that add support for the footnote syntax
+enabled by GFM to [`mdast-util-from-markdown`][mdast-util-from-markdown] and
+[`mdast-util-to-markdown`][mdast-util-to-markdown].
+
+GFM footnotes were [announced September 30, 2021][post] but are not specified.
+Their implementation on github.com is currently buggy.
+The bugs have been reported on [`cmark-gfm`][cmark-gfm].
 
 ## When to use this
 
-Use [`mdast-util-gfm`][mdast-util-gfm] if you want all of GFM.
-Use this otherwise.
+These tools are all rather low-level.
+In most cases, you’d want to use [`remark-gfm`][remark-gfm] with remark instead.
 
-## Install
+When you are working with syntax trees and want all of GFM, use
+[`mdast-util-gfm`][mdast-util-gfm] instead.
+
+When working with `mdast-util-from-markdown`, you must combine this package with
+[`micromark-extension-gfm`][extension].
+
+This utility does not handle how markdown is turned to HTML.
+That’s done by [`mdast-util-to-hast`][mdast-util-to-hast].
+If your content is not in English, you should configure that utility.
 
-This package is [ESM only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c):
-Node 12+ is needed to use it and it must be `import`ed instead of `require`d.
+## Install
 
-[npm][]:
+This package is [ESM only][esm].
+In Node.js (version 12.20+, 14.14+, or 16.0+), install with [npm][]:
 
 ```sh
 npm install mdast-util-gfm-footnote
 ```
 
+In Deno with [`esm.sh`][esmsh]:
+
+```js
+import {gfmFootnoteFromMarkdown, gfmFootnoteToMarkdown} from 'https://esm.sh/mdast-util-gfm-footnote@1'
+```
+
+In browsers with [`esm.sh`][esmsh]:
+
+```html
+<script type="module">
+  import {gfmFootnoteFromMarkdown, gfmFootnoteToMarkdown} from 'https://esm.sh/mdast-util-gfm-footnote@1?bundle'
+</script>
+```
+
 ## Use
 
-Say our module, `example.js`, looks as follows:
+Say our document `example.md` contains:
+
+```markdown
+Hi![^1]
+
+[^1]: big note
+```
+
+…and our module `example.js` looks as follows:
 
 ```js
+import fs from 'node:fs/promises'
 import {fromMarkdown} from 'mdast-util-from-markdown'
 import {toMarkdown} from 'mdast-util-to-markdown'
 import {gfmFootnote} from 'micromark-extension-gfm-footnote'
 import {gfmFootnoteFromMarkdown, gfmFootnoteToMarkdown} from 'mdast-util-gfm-footnote'
 
-const doc = 'Hi![^1]\n\n[^1]: big note'
+const doc = await fs.readFile('example.md')
 
 const tree = fromMarkdown(doc, {
   extensions: [gfmFootnote()],
@@ -61,7 +109,7 @@ const out = toMarkdown(tree, {extensions: [gfmFootnoteToMarkdown()]})
 console.log(out)
 ```
 
-Now, running `node example` yields:
+…now running `node example.js` yields (positional info removed for brevity):
 
 ```js
 {
@@ -94,34 +142,155 @@ Hi\![^1]
 
 ## API
 
+This package exports the identifiers `gfmFootnoteFromMarkdown` and
+`gfmFootnoteToMarkdown`.
+There is no default export.
+
 ### `gfmFootnoteFromMarkdown()`
 
+Function that can be called to get an extension for
+[`mdast-util-from-markdown`][mdast-util-from-markdown].
+
 ### `gfmFootnoteToMarkdown()`
 
-Support footnotes.
-The exports are functions that can be called to get extensions, respectively
-for [`mdast-util-from-markdown`][from-markdown] and
-[`mdast-util-to-markdown`][to-markdown].
+Function that can be called to get an extension for
+[`mdast-util-to-markdown`][mdast-util-to-markdown].
+
+## Syntax tree
+
+The following interfaces are added to **[mdast][]** by this utility.
+
+### Nodes
+
+#### `FootnoteDefinition`
+
+```idl
+interface FootnoteDefinition <: Parent {
+  type: "footnoteDefinition"
+  children: [FlowContent]
+}
+
+FootnoteDefinition includes Association
+```
+
+**FootnoteDefinition** ([**Parent**][dfn-parent]) represents content relating
+to the document that is outside its flow.
+
+**FootnoteDefinition** can be used where [**flow**][dfn-flow-content] content is
+expected.
+Its content model is also [**flow**][dfn-flow-content] content.
+
+**FootnoteDefinition** includes the mixin
+[**Association**][dfn-mxn-association].
+
+**FootnoteDefinition** should be associated with
+[**FootnoteReferences**][dfn-footnote-reference].
+
+For example, the following markdown:
+
+```markdown
+[^alpha]: bravo and charlie.
+```
+
+Yields:
+
+```js
+{
+  type: 'footnoteDefinition',
+  identifier: 'alpha',
+  label: 'alpha',
+  children: [{
+    type: 'paragraph',
+    children: [{type: 'text', value: 'bravo and charlie.'}]
+  }]
+}
+```
+
+#### `FootnoteReference`
+
+```idl
+interface FootnoteReference <: Node {
+  type: "footnoteReference"
+}
+
+FootnoteReference includes Association
+```
+
+**FootnoteReference** ([**Node**][dfn-node]) represents a marker through
+association.
+
+**FootnoteReference** can be used where
+[**static phrasing**][dfn-static-phrasing-content] content is expected.
+It has no content model.
+
+**FootnoteReference** includes the mixin [**Association**][dfn-mxn-association].
+
+**FootnoteReference** should be associated with a
+[**FootnoteDefinition**][dfn-footnote-definition].
+
+For example, the following markdown:
+
+```markdown
+[^alpha]
+```
+
+Yields:
+
+```js
+{
+  type: 'footnoteReference',
+  identifier: 'alpha',
+  label: 'alpha'
+}
+```
+
+### Content model
+
+#### `FlowContent` (GFM footnotes)
+
+```idl
+type FlowContentGfm = FootnoteDefinition | FlowContent
+```
+
+#### `StaticPhrasingContent` (GFM footnotes)
+
+```idl
+type StaticPhrasingContentGfm =
+  FootnoteReference | StaticPhrasingContent
+```
+
+## Types
+
+This package is fully typed with [TypeScript][].
+It does not export additional types.
+
+The `FootnoteDefinition` and `FootnoteReference` node types are supported in
+`@types/mdast` by default.
+
+## Compatibility
+
+Projects maintained by the unified collective are compatible with all maintained
+versions of Node.js.
+As of now, that is Node.js 12.20+, 14.14+, and 16.0+.
+Our projects sometimes work with older versions, but this is not guaranteed.
+
+This plugin works with `mdast-util-from-markdown` version 1+ and
+`mdast-util-to-markdown` version 1+.
 
 ## Related
 
-*   [`remarkjs/remark`][remark]
-    — markdown processor powered by plugins
 *   [`remarkjs/remark-gfm`][remark-gfm]
     — remark plugin to support GFM
-*   [`micromark/micromark`][micromark]
-    — the smallest commonmark-compliant markdown parser that exists
+*   [`syntax-tree/mdast-util-gfm`][mdast-util-gfm]
+    — same but all of GFM (autolink literals, footnotes, strikethrough, tables,
+    tasklists)
 *   [`micromark/micromark-extension-gfm-footnote`][extension]
     — micromark extension to parse GFM footnotes
-*   [`syntax-tree/mdast-util-from-markdown`][from-markdown]
-    — mdast parser using `micromark` to create mdast from markdown
-*   [`syntax-tree/mdast-util-to-markdown`][to-markdown]
-    — mdast serializer to create markdown from mdast
 
 ## Contribute
 
-See [`contributing.md` in `syntax-tree/.github`][contributing] for ways to get
-started.
+See [`contributing.md`][contributing] in [`syntax-tree/.github`][health] for
+ways to get started.
 See [`support.md`][support] for ways to get help.
 
 This project has a [code of conduct][coc].
@@ -162,30 +331,54 @@ abide by its terms.
 
 [npm]: https://docs.npmjs.com/cli/install
 
+[esm]: https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c
+
+[esmsh]: https://esm.sh
+
+[typescript]: https://www.typescriptlang.org
+
 [license]: license
 
 [author]: https://wooorm.com
 
-[contributing]: https://github.com/syntax-tree/.github/blob/HEAD/contributing.md
+[health]: https://github.com/syntax-tree/.github
+
+[contributing]: https://github.com/syntax-tree/.github/blob/main/contributing.md
 
-[support]: https://github.com/syntax-tree/.github/blob/HEAD/support.md
+[support]: https://github.com/syntax-tree/.github/blob/main/support.md
 
-[coc]: https://github.com/syntax-tree/.github/blob/HEAD/code-of-conduct.md
+[coc]: https://github.com/syntax-tree/.github/blob/main/code-of-conduct.md
 
 [mdast]: https://github.com/syntax-tree/mdast
 
 [mdast-util-gfm]: https://github.com/syntax-tree/mdast-util-gfm
 
-[remark]: https://github.com/remarkjs/remark
-
 [remark-gfm]: https://github.com/remarkjs/remark-gfm
 
-[from-markdown]: https://github.com/syntax-tree/mdast-util-from-markdown
-
-[to-markdown]: https://github.com/syntax-tree/mdast-util-to-markdown
+[extension]: https://github.com/micromark/micromark-extension-gfm-footnote
 
-[micromark]: https://github.com/micromark/micromark
+[gfm]: https://github.github.com/gfm/
 
-[extension]: https://github.com/micromark/micromark-extension-gfm-footnote
+[cmark-gfm]: https://github.com/github/cmark-gfm
 
 [post]: https://github.blog/changelog/2021-09-30-footnotes-now-supported-in-markdown-fields/
+
+[mdast-util-from-markdown]: https://github.com/syntax-tree/mdast-util-from-markdown
+
+[mdast-util-to-markdown]: https://github.com/syntax-tree/mdast-util-to-markdown
+
+[mdast-util-to-hast]: https://github.com/syntax-tree/mdast-util-to-hast
+
+[dfn-parent]: https://github.com/syntax-tree/mdast#parent
+
+[dfn-mxn-association]: https://github.com/syntax-tree/mdast#association
+
+[dfn-node]: https://github.com/syntax-tree/unist#node
+
+[dfn-flow-content]: #flowcontent-gfm-footnotes
+
+[dfn-static-phrasing-content]: #staticphrasingcontent-gfm-footnotes
+
+[dfn-footnote-reference]: #footnotereference
+
+[dfn-footnote-definition]: #footnotedefinition