Add CONTRIBUTING file.
Add CONTRIBUTING file.

Change-Id: I07519e58a8ee2b1dc01e9882e50c68db5fa6ebab

larsks/python-heatclient,metacloud/python-heatclient,ecerulm/python-heatclient,rdo-management/python-heatclient,openstack/python-heatclient,jasondunsmore/python-heatclient,tcpcloud/python-heatclient,ecerulm/python-heatclient,varunarya10/python-heatclient,metacloud/python-heatclient,jasondunsmore/python-heatclient,larsks/python-heatclient,tcpcloud/python-heatclient,citrix-openstack-build/python-heatclient,citrix-openstack-build/python-heatclient,varunarya10/python-heatclient,rdo-management/python-heatclient