add CONTRIBUTING.rst

The module has no CONTRIBUTING.rst file, so add it.

Change-Id: I5cb55243527003b3f8a8a000a1610f0ab7e6772b

CONTRIBUTING.rst

@@ -0,0 +1,16 @@
+If you would like to contribute to the development of OpenStack,
+you must follow the steps in this page:
+
+   http://docs.openstack.org/infra/manual/developers.html
+
+Once those steps have been completed, changes to OpenStack
+should be submitted for review via the Gerrit tool, following
+the workflow documented at:
+
+   http://docs.openstack.org/infra/manual/developers.html#development-workflow
+
+Pull requests submitted through GitHub will be ignored.
+
+Bugs should be filed on Launchpad, not GitHub:
+
+   https://bugs.launchpad.net/mistral