doc: readme on how to build the doc

README.md

@@ -102,6 +102,22 @@ nix-shell -A test --command "pytest"
 
 For more information about the tests, read the [test README](test/README.md)
 
+## Build the documentation
+Batmen documentation is hosted on readthedocs: [Batmen doc].
+It is automatically built and updated whenever there is a push on Batmen's `master` branch.
+
+To build the doc manually, we provide a dedicated nix shell.
+From Batmen root directory:
+
+```shell
+cd docs/
+nix-shell -A doc
+make html
+```
+
+The doc is generated inside the folder `docs/_build` and can be viewed on the browser by opening the file `docs/_build/html/index.html`. 
+
 [Nix]: https://nixos.org/nix/
 [Batsim]: https://batsim.org/
-[batsched]: https://framagit.org/batsim/batsched
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+[batsched]: https://framagit.org/batsim/batsched
+[Batmen doc]: https://batmen.readthedocs.io/en/latest/
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