Add CONTRIBUTING.rst
Add CONTRIBUTING.rst

There is no CONTRIBUTING.rst file, the patch will add it.

Change-Id: Ifcdc30b47b5fb668bbca2e974d8cff63a938addb

dougwig/x-neutron-fwaas,openstack/neutron-fwaas,citrix-openstack-build/neutron-fwaas,gaolichuang/neutron-fwaas,gaolichuang/neutron-fwaas,openstack/neutron-fwaas