Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -45,6 +45,14 @@ Yields:
 Transform a [HAST Node][node] to an `ASTNode` according to the default
 Parse5 adapter.
 
+## Contribute
+
+See [`contribute.md` in `syntax-tree/hast`][contribute] for ways to get
+started.
+
+This organisation has a [Code of Conduct][coc].  By interacting with this
+repository, organisation, or community you agree to abide by its terms.
+
 ## License
 
 [MIT][license] © [Titus Wormer][author]
@@ -70,3 +78,7 @@ Parse5 adapter.
 [ast]: https://github.com/inikulin/parse5/wiki/Documentation
 
 [node]: https://github.com/syntax-tree/hast#ast
+
+[contribute]: https://github.com/syntax-tree/hast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/hast/blob/master/code-of-conduct.md