[tor-dev] Note on my GPG key update

Hi! Please see https://people.torproject.org/~nickm/key-transition-statement-2.txt for information on my updated GPG key. Thanks! (I wouldn't ordinarily send this here, but this new key signs the Tor source and tags these days, so it's a good idea to make sure people know about it. The most recent releases were signed with both my old key and my new key: this is the last time that I'm planning to be doing that, however, since it seemed to confuse a lot of software.)