Update README for xenial
Update README for xenial

Documented workaround for readonly file system error in virtualbox.

Change-Id: I604dadcebce8a04db47eec57688395650dfe9626

openstack/monasca-api,openstack/monasca-api,openstack/monasca-api,stackforge/monasca-api,stackforge/monasca-api,stackforge/monasca-api,stackforge/monasca-api,sapcc/monasca-api,sapcc/monasca-api,sapcc/monasca-api