ReST-ize the CONTRIBUTING guide
It used to have Markdown markup but without an extension this wasn't rendered by
any tool (including Github). ReST-ize for consistency with the README and since
it's the more common markup format for Python.
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
new file mode 100644
index 0000000..822cadb
--- /dev/null
+++ b/CONTRIBUTING.rst
@@ -0,0 +1,40 @@
+Want to contribute? Great! First, read this page (including the small print at the end).
+
+Before you contribute
+---------------------
+
+Before we can use your code, you must sign the `Google Individual Contributor
+License Agreement
+<https://developers.google.com/open-source/cla/individual?csw=1>`_ (CLA), which
+you can do online. The CLA is necessary mainly because you own the
+copyright to your changes, even after your contribution becomes part of our
+codebase, so we need your permission to use and distribute your code. We also
+need to be sure of various other things—for instance that you'll tell us if you
+know that your code infringes on other people's patents. You don't have to sign
+the CLA until after you've submitted your code for review and a member has
+approved it, but you must do it before we can put your code into our codebase.
+Before you start working on a larger contribution, you should get in touch with
+us first through the issue tracker with your idea so that we can help out and
+possibly guide you. Coordinating up front makes it much easier to avoid
+frustration later on.
+
+Code reviews
+------------
+
+All submissions, including submissions by project members, require review. We
+use Github pull requests for this purpose.
+
+YAPF coding style
+-----------------
+
+YAPF follows the `Google Python Style Guide
+<https://google-styleguide.googlecode.com/svn/trunk/pyguide.html>`_, with the
+exception of using 2 spaces for indentation rather than 4. YAPF was initially
+developed internally and 2 spaces is still the prevalent Python style within
+Google.
+
+Small print
+-----------
+
+Contributions made by corporations are covered by a different agreement than
+the one above, the Software Grant and Corporate Contributor License Agreement.