Add link to readme at bottom of docs.openstack.org/developer/hacking/

The readme has important information so we want to make it discoverable
at http://docs.openstack.org/developer/hacking/
Remove 'Further Reading' section in HACKING.rst since it's already in
the README.rst which is now linked at the bottom of
http://docs.openstack.org/developer/hacking/

Change-Id: I45c51494382283003f61fc416953a576efa7db60
Merge "Add link to readme at bottom of docs.openstack.org/developer/hacking/"