Update manual to explain globe icon colours

As mentioned here:

Sorry, we haven't updated the manual yet to explain the icon colours. Orange means Briar is connecting to the Tor network - messages can't be sent or received via the Internet in this state. Green means Briar's connected to the Tor network and it should be possible to send or receive messages (although it may take a minute or two before your contacts notice that you're connected and try to send any messages that were waiting to send).