Add a note about git commit message style.

Update contributing.rst to point to Tim Pope's suggestions
for how to write git commit messages.

Change-Id: Idcbece3e7576f54794d8a107aa2c5aa48dc4ddf7

docs/source/contributing.rst

diff --git a/docs/source/contributing.rst b/docs/source/contributing.rst
index 88c97af..aed509a 100644
--- a/docs/source/contributing.rst
+++ b/docs/source/contributing.rst
@@ -104,6 +104,9 @@
    like. Commit your changes as one patch. When you commit, the Gerrit
    hook will add a `Change-Id:` line as the last line of the commit.
 
+   Make sure that your commit message is formatted in the `50/72 style
+   <http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html>`_.
+
 2. Push your changes to the Ceres Gerrit instance:
 
    .. code-block:: bash