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.
Please remove any reference to btrfs. Whether ZFS is better than btrfs is irrelevant, may differ by use case, and we're not a ZFS company (we may well add support for btrfs in the future).