move incentives.txt over to torspec proposals or ideas
https://gitweb.torproject.org/tor.git/blob/HEAD:/doc/contrib/incentives.txt is still in Tor's git.
It probably should have been called a proposal long ago. But I guess it missed that boat.
Should it go into the torspec proposals/ideas/ directory? Or be made a proposal anyway, for completeness? It is mostly of historical interest now (since so many papers have been written on the topic since I wrote this text). But it still has some points that I think are not clearly made by the so-many-papers.