Add a CONTRIBUTING file.

GitHub likes repos to have a CONTRIBUTING file.  It will use this to
point people to the right place for how to contribute to this project.

Change-Id: I52ce1489527d2c7d68776c786731af4b2d675c38

CONTRIBUTING.rst

@@ -0,0 +1,17 @@
+If you would like to contribute to the development of OpenStack,
+you must follow the steps in the "If you're a developer, start here"
+section of this page:
+
+   http://wiki.openstack.org/HowToContribute
+
+Once those steps have been completed, changes to OpenStack
+should be submitted for review via the Gerrit tool, following
+the workflow documented at:
+
+   http://wiki.openstack.org/GerritWorkflow
+
+Pull requests submitted through GitHub will be ignored.
+
+Bugs should be filed on Launchpad, not GitHub:
+
+   https://bugs.launchpad.net/nova

README.rst

@@ -50,9 +50,7 @@ developer focused documentation is available at:
 
    http://nova.openstack.org/
 
-Changes to OpenStack Nova should be submitted for review via
-the Gerrit tool, following the workflow documented at:
-
-   http://wiki.openstack.org/GerritWorkflow
+For information on how to contribute to Nova, please see the
+contents of the CONTRIBUTING.rst file.
 
 -- End of broadcast