Add HACKING.rst

Add HACKING.rst but instead of copying the file around point to the new
cannonical location for it
https://github.com/openstack-dev/hacking/blob/master/HACKING.rst

Also expand out the flake8 H series ignores, so they can be enabled one
by one.

Change-Id: I6ec39d00c3beabbc3e661754bac6e43a0fb351d6
Merge "Add HACKING.rst"