Update CONTRIBUTING.rst
Update CONTRIBUTING.rst

Potential contributors are still not getting the message about our
disallowed minor code changes, so adding a paragraph to CONTRIBUTING.rst
to mention this up front.

Change-Id: I837590bbb83cfba83433c7c4625a2ca5870fa179

rajalokan/glance,stevelle/glance,rajalokan/glance,openstack/glance,openstack/glance,stevelle/glance,openstack/glance