Update rules.md: Concerning the list of players

I'm not entirely sure I've done this right.  I forked, edited my own copy, and submited a pull request on the diff.  Is that right?

As for the reasoning behind the proposed amendment, my thinking is that (1) it would be good to have a formal process to add new players in the future, and (2) it would be good to have some formal protection against loss of playerhood at the hand of a capricious git master.