Add a sentence about using mock rather than mox for unit tests.
Change-Id: If3c175d986f3ebed6d949df5c473e0cf9177ef6e
submitted bug fix does have a unit test, be sure to add a new one that fails
without the patch and passes with the patch.
+Cinder is transitioning to use mock, rather than mox, and so new tests should
+use mock only.
+
For more information on creating unit tests and utilizing the testing
infrastructure in OpenStack Cinder, please read the Cinder testing
`README.rst <https://github.com/openstack/cinder/blob/master/cinder/testing/README.rst>`_.