Allow to use buttervolume without any BTRFS partition

[ccomb]

One thing that can prevent people from using Buttervolume is that it gives the impression that one needs to have a dedicated BTRFS partition. However it is possible to format a single file and mount it as /var/lib/buttervolume. To ease the initial setup, either add something like a buttervolume init command, or explain how to do in the README file.