Docs faq 348

Resolves: https://github.com/ClusterHQ/flocker/issues/348

Particularly interested in @lukemarsden feedback since he is hearing these questions all the time.
It seems very confusing to have a FAQ about a feature that isn't mentioned in any of the documentation. If you want to put it elsewhere that would be fine. Wouldn't a discussion of ZFS's capabilities (cloning included) be more helpful in general? We don't really talk about ZFS anywhere in the docs now even though it's a key technology and reason we can do the things we're doing, and would present location to talk about future capabilities.