Add a bit more detail to README

Author: jmert

README.md

@@ -1,6 +1,44 @@
 # UnixMmap
 
-[![Build Status](https://github.com/jmert/UnixMmap.jl/workflows/CI/badge.svg)](https://github.com/jmert/UnixMmap.jl/actions)
+| **Build Status**                                             |
+|:------------------------------------------------------------:|
+| [![][ci-img]][ci-url]                                        |
 
 `UnixMmap` is an alternative to the Julia standard library's `Mmap`, with the purpose of
-exposing the Unix memory-mapping interface.
+exposing the Unix memory-mapping interface.
+
+### Installation and usage
+
+This library is **not** registered in Julia's [General registry][General.jl],
+so the package must be installed either by cloning it directly:
+
+```
+(@v1.6) pkg> add https://github.com/jmert/UnixMmap.jl
+```
+
+or by making use of my [personal registry][Registry.jl]:
+
+```
+(@v1.6) pkg> registry add https://github.com/jmert/Registry.jl
+(@v1.6) pkg> add UnixMmap
+```
+
+After installing, just load like any other Julia package:
+
+```
+julia> using UnixMmap
+```
+
+[docs-stable-img]: https://img.shields.io/badge/docs-stable-blue.svg
+[docs-stable-url]: https://jmert.github.io/UnixMmap.jl/stable
+[docs-dev-img]: https://img.shields.io/badge/docs-dev-blue.svg
+[docs-dev-url]: https://jmert.github.io/UnixMmap.jl/dev
+
+[ci-img]: https://github.com/jmert/UnixMmap.jl/workflows/CI/badge.svg
+[ci-url]: https://github.com/jmert/UnixMmap.jl/actions
+
+[codecov-img]: https://codecov.io/gh/jmert/UnixMmap.jl/branch/master/graph/badge.svg
+[codecov-url]: https://codecov.io/gh/jmert/UnixMmap.jl
+
+[General.jl]: https://github.com/JuliaRegistries/General
+[Registry.jl]: https://github.com/jmert/Registry.jl