Add contributing file

Change-Id: Iae015eae0931c5cd0d77990964fa57c5594e4bb6

CONTRIBUTING.rst

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+.. _contributing:
+
+=============================================
+Contributing to ansible-collections-openstack
+=============================================
+
+If you're interested in contributing to the ansible-collections-openstack project,
+the following will help get you started.
+
+Developer Workflow
+------------------
+
+OpenStack uses OpenDev for it's development, and patches are submitted to
+`OpenDev Gerrit`_. Please read `DeveloperWorkflow`_ before sending your
+first patch for review.
+
+Pull requests submitted through GitHub will be ignored.
+
+.. seealso::
+
+   * https://wiki.openstack.org/wiki/How_To_Contribute
+   * https://wiki.openstack.org/wiki/CLA
+
+.. _OpenDev Gerrit: https://review.opendev.org/
+.. _DeveloperWorkflow: https://docs.openstack.org/infra/manual/developers.html#development-workflow
+
+Project Hosting Details
+-----------------------
+
+Bug tracker
+    https://storyboard.openstack.org/#!/project/openstack/ansible-collections-openstack
+
+Mailing list (prefix subjects with ``[ansible]`` for faster responses)
+    http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-discuss
+
+Code Hosting
+    https://opendev.org/openstack/ansible-collections-openstack
+
+Code Review
+    https://review.opendev.org/#/q/status:open+project:openstack/ansible-collections-openstack,n,z