Add CONTRIBUTING.md
Add CONTRIBUTING.md

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

Change-Id: Idc0a9ae22f2b315375b660263c7602e3ca02d56b

VyacheslavHashov/python-swiftclient,krnflake/python-hubicclient,sohonetlabs/python-swiftclient,pratikmallya/python-swiftclient,JioCloud/python-swiftclient,VyacheslavHashov/python-swiftclient,sohonetlabs/python-swiftclient,iostackproject/IO-Bandwidth-Differentiation-Client,JioCloud/python-swiftclient,openstack/python-swiftclient,pratikmallya/python-swiftclient,openstack/python-swiftclient,ironsmile/python-swiftclient,jeseem/python-swiftclient,iostackproject/IO-Bandwidth-Differentiation-Client,varunarya10/python-swiftclient,jeseem/python-swiftclient,ironsmile/python-swiftclient,varunarya10/python-swiftclient