Adds Cloud Files API/Providers to README

The subject says it all! 

This can be merged once the main Rackspace Cloud Files API/US provider [PR 79](https://github.com/jclouds/jclouds-labs-openstack/pull/79) is merged, and secondary dependent pull request is submitted/merged for the Cloud Files UK provider.

I will follow up on this once those PRs have been merged to master.