Add CONTRIBUTING.rst
Add CONTRIBUTING.rst

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

Change-Id: I129af2c2301c47df5619f4fe279127b68f974098

ramineni/myclient,rdo-management/python-ironicclient,varunarya10/python-ironicclient,redhat-openstack/python-ironicclient,NaohiroTamura/python-ironicclient,openstack/python-ironicclient,bcrochet/python-ironicclient,CiscoUcs/UCS-python-ironicclinet,NaohiroTamura/python-ironicclient,openstack/python-ironicclient,JioCloud/python-ironicclient