Add stackrc comments to HACKING

Change-Id: I46ff885184a2b5b71caca905c27f28d8b1304011
Merge branch 'fix-tempest-flavors' into dt-test
Merge branch 'ssl-proxy-cinder' into dt-test
Merge branch 'tls-split' into dt-test
Merge branch 'extras-dir' into dt-test
Merge branch 'hacking' into dt-test