mk: add rftg-license for games/rftg


(wiz)

The original game Race for the Galaxy is designed by Tom Lehmann
and published by Rio Grande Games.

The source code is copyrighted and is placed under the GNU General
Public License, version 2 (GPLv2).

Rio Grande Games holds the copyrights for the images. Permission
to distribute the card and goal images has been granted by Rio
Grande Games. The image files may not be unpacked or redistributed
without this notice, or used for any other purpose.