Add CONTRIBUTING.rst
Add CONTRIBUTING.rst

There is no CONTRIBUTING.rst file, so i add it.

Change-Id: Ib017bc49623f2238a32f316af02cbc22b6be7a5b

darren-wang/gl,scripnichenko/glance,darren-wang/gl,JioCloud/glance,akash1808/glance,rajalokan/glance,kfwang/Glance-OVA-OVF,stevelle/glance,wkoathp/glance,rajalokan/glance,wkoathp/glance,tanglei528/glance,vuntz/glance,redhat-openstack/glance,saeki-masaki/glance,jumpstarter-io/glance,dims/glance,openstack/glance,takeshineshiro/glance,kfwang/Glance-OVA-OVF,openstack/glance,paramite/glance,vuntz/glance,ozamiatin/glance,paramite/glance,akash1808/glance,saeki-masaki/glance,ozamiatin/glance,klmitch/glance,openstack/glance,klmitch/glance,sigmavirus24/glance,takeshineshiro/glance,dims/glance,jumpstarter-io/glance,sigmavirus24/glance,scripnichenko/glance,redhat-openstack/glance,stevelle/glance,JioCloud/glance,tanglei528/glance