Add a note about using "Differential Revision:" in commit messages

I noticed this convention from the commit logs.  It seems like it
would be useful to document it, to encourage other committers to link
back to code reviews in their commits.

Differential Revision: http://llvm-reviews.chandlerc.com/D2678

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201160 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Phabricator.rst

@@ -94,6 +94,26 @@ when a review changes state, for example by clicking "Accept Revision" in
 the web interface. Thus, please type LGTM into the comment box to accept
 a change from Phabricator.
 
+Committing a change
+-------------------
+
+When committing an LLVM change that has been reviewed using
+Phabricator, the convention is for the commit message to end with the
+line:
+
+::
+
+  Differential Revision: <URL>
+
+where ``<URL>`` is the URL for the code review, starting with
+``http://llvm-reviews.chandlerc.com/``.
+
+Note that Arcanist will add this automatically.
+
+This allows people reading the version history to see the review for
+context.  This also allows Phabricator to detect the commit, close the
+review, and add a link from the review to the commit.
+
 Status
 ------