Add proper README for how to develop on hacking

Move parts of HACKING to README, along with how to add rules, how to
version. This is the first pass at the README, it's expected to continue
to grow

Change-Id: I2dcbc0ae8a65f67c64cf406ac2efc6ee4a305ee4
Reviewed-on: https://review.openstack.org/30942
Reviewed-by: Mark McLoughlin <markmc@redhat.com>
Reviewed-by: Sean Dague <sean@dague.net>
Approved: Mark McLoughlin <markmc@redhat.com>
Tested-by: Jenkins