Merge "Add CONTRIBUTING.rst"

CONTRIBUTING.rst

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+If you would like to contribute to the development of OpenStack,
+you must follow the steps documented at:
+
+   http://docs.openstack.org/developer/horizon/contributing.html
+
+   or https://wiki.openstack.org/wiki/How_To_Contribute#If_you.27re_a_developer
+
+Once those steps have been completed, changes to OpenStack
+should be submitted for review via the Gerrit tool, following
+the workflow documented at:
+
+   https://wiki.openstack.org/GerritWorkflow
+
+Pull requests submitted through GitHub will be ignored.
+
+Bugs should be filed on Launchpad, not GitHub:
+
+   https://bugs.launchpad.net/horizon