Add SDL_GameController config information

This adds a text file with information about an environment variable that can be used by SDL2 to automatically configure the GameCube controller for games that use SDL_GameController.

This config line was obtained using the controllermap.c test from SDL's source repository:

https://hg.libsdl.org/SDL/file/default/test/controllermap.c

This should be the same output from the config menu in Steam Big Picture mode, though at the moment this menu crashes on my PC... not sure why. In any case, this might be easier for those who run into this problem, or want to have their games configured without having to use Steam.

This configuration has been tested and verified with two controllers (separate and combined) with all of my SDL2-based games (Capsized onward):

http://www.flibitijibibo.com/index.php?page=Portfolio/Ports
I've noticed the adapter seems to never really reach the full 0 or 255 on the axis data it returns. If this causes issues with SDL it could be fixed. I can also make the triggers be [0, 32767] instead of [-32768, 32767] as well.