We do not actually support persistence

See https://github.com/alphagov/vcloud-edge_gateway/issues/36. I am removing it from the README for now and have raised a bug in Pivotal https://www.pivotaltracker.com/story/show/68445956 to add support later.
Clarify what is currently supported
Merge pull request #38 from alphagov/remove-persistence-from-readme

We do not actually support persistence