README: Document how to build the docs locally

README.md

@@ -151,6 +151,29 @@ build to keep the Docker image up-to-date.
 
 [docker-base]: https://github.com/nextstrain/docker-base
 
+### Building documentation
+
+[Documentation](https://nextstrain-augur.readthedocs.io) is built using
+[Sphinx](http://sphinx-doc.org/) and hosted on [Read The
+Docs](https://readthedocs.org/).  Versions of the documentation for each augur
+release and git branch are available and preserved.
+
+Building the documentation locally is useful to test changes.  First, make sure
+you have the development dependencies of augur installed:
+
+    pip install '.[dev]'
+
+This installs packages listed in the `dev` section of `extras_require` in
+_setup.py_ (in addition to any normal augur dependencies if necessary).
+
+Then build the HTML output format by running:
+
+    make -C docs html
+
+You can see other available formats by running:
+
+    make -C docs help
+
 ## License and copyright
 
 Copyright 2014-2019 Trevor Bedford and Richard Neher.