Add instructions on generating documentation.

Signed-off-by: Evan Anderson <evan.k.anderson@gmail.com>

docs/devel/generate-docs.rst

@@ -0,0 +1,31 @@
+=====================
+Generating Documentation
+=====================
+
+The documentation uses `Sphinx <http://www.sphinx-doc.org/>` (via
+`Read The Docs <https://readthedocs.org/>`) to generate markdown from
+`reStructured Text <http://www.sphinx-doc.org/en/master/usage/restructuredtext/index.html>`.
+
+Installation instructions
+=========================
+
+To install the sphinx tools, you'll need ``python`` (and ``pip``) installed.::
+
+.. code-block: shell
+
+   pip install --user -r requirements.txt
+
+Generating documentation locally
+================================
+
+You can generate the documentation locally with the following command:
+
+.. code-block: shell
+
+   make html
+
+This will create documentation in the ``_build`` directory which you can
+open with your browser.
+
+Note that you do not need to add these files to your git client, as
+*Read The Docs* will generate the HTML on the fly.