Improve Docker README file
Improve Docker README file

Change-Id: I208c4b4957a40616a9f66c74bcd307630747e1c9

stackforge/monasca-persister,openstack/monasca-persister,openstack/monasca-persister,stackforge/monasca-persister,stackforge/monasca-persister,openstack/monasca-persister