Update links in README

* github to git.openstack.org
* ML openstack to openstack-dev as that is where development discussions
  happen
* nova.openstack.org to docs.openstack.org/developer/nova as that is
  the nova.openstack.org just redirects to there

Change-Id: I884bb2dff3d026618c0bce202f938c7e87bffe04
Merge "Update links in README"