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
Huh, for some reason the HAT1 values never show up in SDL_Joystick's events. I changed it to HAT0 and it did start giving us SDL_JOYHATMOTION events, but those values didn't seem to make a whole lot of sense... so I changed it again to ABS_Z and ABS_RZ, and those at least do make sense, though it starts at -32767 and ends at around 20383... odd, but it's something. Will have to see if there's a nice way to morph that into [0, 32767].