Add a link from HACKING to wiki GitCommitMessages page

The HACKING file contains a short example of an effective
commit message. For reasons for space, it cannot describe
the rationale behind this example. It also does not have
space to describe how to split up a patch into a series
of commits. Add a link from the HACKING file to the wiki

   http://wiki.openstack.org/GitCommitMessages

where further information can be found

Change-Id: I34d44485486b623b11743106f09d5ef631d35888

HACKING.rst

@@ -234,3 +234,9 @@ readable. Follow these guidelines:
   message: a blank line followed by a 'Change-Id'. This is important
   to correlate this commit with a specific review in Gerrit, and it
   should not be modified.
+
+For further information on constructing high quality commit messages,
+and how to split up commits into a series of changes, consult the
+project wiki:
+
+   http://wiki.openstack.org/GitCommitMessages