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

[mseaborn]

Summary

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.

[mseaborn]

Changed wording; mentioned Arcanist.

[mseaborn]

Closed by commit rL201160 (authored by @mseaborn).

llvm/trunk/docs/Phabricator.rst

 if you have a question, drop by on #llvm in IRC to get help.
 
 Note that as e-mail is the system of reference for code reviews, and some
 people prefer it over a web interface, we do not generate automated mail
 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
 ------
 
 Currently, we're testing Phabricator for use with Clang/LLVM. Please let us
 know whether you like it and what could be improved!
 
 .. _LLVM's Phabricator: http://llvm-reviews.chandlerc.com
 .. _Code Repository Browser: http://llvm-reviews.chandlerc.com/diffusion/
 .. _Arcanist Quick Start: http://www.phabricator.com/docs/phabricator/article/Arcanist_Quick_Start.html
 .. _Arcanist User Guide: http://www.phabricator.com/docs/phabricator/article/Arcanist_User_Guide.html