Add clarity to CONTRIBUTING
Add clarity to CONTRIBUTING

Some feedback was provided after https://github.com/rcbops/rpc-openstack/pull/1848
merged. This commit updates some information about adding tempest tests,
and updates the URLs to use a specific SHA of the script

Connects Connects https://github.com/rcbops/rpc-openstack/issues/1800

jacobwagner/rpc-openstack,prometheanfire/rpc-openstack,cloudnull/rpc-openstack,hughsaunders/rpc-openstack,jacobwagner/rpc-openstack,major/rpc-openstack,rcbops/rpc-openstack,mancdaz/rpc-openstack,robb-romans/rpc-openstack,git-harry/rpc-openstack,cloudnull/rpc-openstack,galstrom21/rpc-openstack,shannonmitchell/rpc-openstack,BjoernT/rpc-openstack,git-harry/rpc-openstack,galstrom21/rpc-openstack,robb-romans/rpc-openstack,rcbops/rpc-openstack,BjoernT/rpc-openstack,major/rpc-openstack,cfarquhar/rpc-openstack,mancdaz/rpc-openstack,cfarquhar/rpc-openstack,shannonmitchell/rpc-openstack,prometheanfire/rpc-openstack,hughsaunders/rpc-openstack