Add a minimum documentation for devstack plugin
Add a minimum documentation for devstack plugin

Related-Bug: #1501795
Related-Bug: #1515104
Change-Id: Id2d37be361b6e675620604abff15e16fa09378b1

stackforge/tap-as-a-service,stackforge/tap-as-a-service,openstack/tap-as-a-service,openstack/tap-as-a-service