Print copyright notice in stdout console on program start

Small detail because GPL recommends printing a small notice on program start.

This may be relevant for Linux packaging, as the README may not be easily discoverable (?)

This prints the following message on program start and in log.txt. It's hidden from view in the game console.

Sonic Robo Blast 2
Copyright (C) 1998-2018 by Sonic Team Junior

This program comes with ABSOLUTELY NO WARRANTY.

This is free software, and you are welcome to redistribute it
and/or modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2 of
the License, or (at your option) any later version.
See the 'LICENSE.txt' file for details.

Sonic the Hedgehog and related characters are trademarks of SEGA.
We do not claim ownership of SEGA's intellectual property used
in this program.

This PR also adds README.txt, etc., to the cmake install.