Import the Git style guide from the wiki

It is already referenced by a couple of pages,
but I forgot to import it.

docs/contributing/git-style-guide.rst

+.. _git-style-guide:
+
+Git style guide
+===============
+
+Various information about how we use Git to develop Software Heritage.
+
+Commits
+-------
+
+Make your commits adhere to this `How to Write a Git Commit Message <http://chris.beams.io/posts/git-commit/>`_
+
+Link commits to tasks
++++++++++++++++++++++
+
+You can reference Phabricator tasks from your commits,
+using a `dedicated syntax <https://secure.phabricator.com/T5132>`_
+When you do so, please put the task action on a separate line,
+so that it is clearly visible.
+
+Make sure commits that are enough to close a bug do so using a line like::
+
+   Closes T123456
+
+If you just want to "ping" a task, updating it with the fact that
+a related commit has been pushed, use::
+
+   Related to T123456
+
+References
+----------
+
+* `special syntax you can use in commit messages to cause effects <https://secure.phabricator.com/T5132>`_

docs/contributing/index.rst

@@ -10,4 +10,5 @@ Contributing
    phabricator
    code-review
    python-style-guide
+   git-style-guide
    sphinx