Added Cloud Files API/Providers to README

The subject says it all!

This PR can be merged once the Rackspace Cloud Files API/US Provider [PR 79](https://github.com/jclouds/jclouds-labs-openstack/pull/79) and another dependent PR to include the Rackspace Cloud Files UK Provider are merged to master.

I will check back once that happens to move this along. Thx!