Change README to include section on changing storage backend

It wasn't immediately obvious to me how to change the storage backend, so I've added it to the README. Feel free to ignore if this is referenced in some other, more logical place. :-)
Can you please adjust `riak_kv` here to something like `node['riak']['config']['riak_kv']['storage_backend']` or `storage_backend`? `riak_kv` itself is an entire section of the overall configuration file.