Add installation instructions (python#2)

Author: ncoghlan

README.rst

@@ -9,3 +9,8 @@ years. That's fine, and you're welcome to do so for other Python community
 projects if you so choose, but please keep in mind that in doing so you're also
 choosing to accept some of the responsibility for maintaining that collective
 trust.
+
+To use the theme either clone it directly with `git`, or else install it
+into your docs build environment via `pip`:
+
+    pip install git+https://github.com/python/python-docs-theme.git#egg=python-docs-theme