Update pointer to HACKING.rst in the readme
Update pointer to HACKING.rst in the readme

When moving HACKING out of doc/source/index.rst to HACKING.rst
README.rst wasn't updated.

Change-Id: Ice2eebe42eedc27e9dec80439216d3c0c9485d01

openstack-dev/hacking,hyakuhei/cleantox,openstack-dev/hacking,zancas/hacking,zancas/hacking,hyakuhei/cleantox