Expand HACKING with commit message guidelines
Expand HACKING with commit message guidelines

Add a 'Commit Messages' section to HACKING describing how a commit
message should be formed.

Change-Id: Ife0d6eab18cc0f07ad8e8c79cb2e4198513236d2

varunarya10/python-glanceclient,ntt-sic/python-glanceclient,JioCloud/python-glanceclient,klmitch/python-glanceclient,mmasaki/python-glanceclient,varunarya10/python-glanceclient,metacloud/python-glanceclient,mmasaki/python-glanceclient,openstack/python-glanceclient,ntt-sic/python-glanceclient,JioCloud/python-glanceclient,alexpilotti/python-glanceclient,metacloud/python-glanceclient,alexpilotti/python-glanceclient,klmitch/python-glanceclient,openstack/python-glanceclient,citrix-openstack-build/python-glanceclient,citrix-openstack-build/python-glanceclient