Sync Readme, these features have already been implemented

Party mode is also implemented but there is this call:
https://github.com/soffes/sonos/blob/master/lib/sonos/system.rb#L40 which seems odd. Removing it does create a party but only for the first two speakers and it throws a `UPnPError`; to be investigated.

For now, the three working items can be removed from the ToDo list.