Rename README to HACKING

In preparation for adding a proper README

Change-Id: I1c3680c952a6a74dacec3e5af2b5a052941100f2
Reviewed-on: https://review.openstack.org/30941
Reviewed-by: Sean Dague <sean@dague.net>
Approved: Mark McLoughlin <markmc@redhat.com>
Reviewed-by: Mark McLoughlin <markmc@redhat.com>
Tested-by: Jenkins