Document how to generate docs

docs/documentation.md

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+Documentation
+=============
+
+Documentation is generated using [mdBook](https://rust-lang.github.io/mdBook/).
+
+Documentation is written in markdown and located in the `docs` directory.
+
+Additional plugins:
+
+- [mdbook-toc](https://github.com/badboy/mdbook-toc)
+
+Install prerequisites
+---------------------
+
+You'll need a moderately recent version of Rust (1.60) at the time of writing.
+Check out [mdbook installation docs](https://rust-lang.github.io/mdBook/guide/installation.html)
+for details.
+
+Install by building from source:
+
+```
+cargo install mdbook mdbook-toc
+```
+
+Generate
+--------
+
+HTML documentation is generated from sources by running:
+
+```
+mdbook build
+```
+
+To run a local server which will rebuild on change:
+
+```
+mdbook serve
+```