Add release notes for 0.12.0
Add release notes for 0.12.0

Change-Id: I7642635ba0326d97ca44f08ca54c1d613f1925f1

metacloud/python-glanceclient,klmitch/python-glanceclient,JioCloud/python-glanceclient,varunarya10/python-glanceclient,metacloud/python-glanceclient,openstack/python-glanceclient,mmasaki/python-glanceclient,klmitch/python-glanceclient,mmasaki/python-glanceclient,varunarya10/python-glanceclient,openstack/python-glanceclient,alexpilotti/python-glanceclient,JioCloud/python-glanceclient,alexpilotti/python-glanceclient