Add HACKING.rst
Add HACKING.rst

Add information about hacking and testing the repository.

Change-Id: I48bc4e6342295a35959cbd796ac0be1fb08a9dd2

openstack/openstack-doc-tools,savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,openstack/openstack-doc-tools