SDL2: Add game controller mapping database file.

resources/SDL_GameControllerDB/LICENSE

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+
+Simple DirectMedia Layer
+Copyright (C) 1997-2013 Sam Lantinga <[email protected]>
+
+This software is provided 'as-is', without any express or implied
+warranty.  In no event will the authors be held liable for any damages
+arising from the use of this software.
+
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it
+freely, subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not
+   claim that you wrote the original software. If you use this software
+   in a product, an acknowledgment in the product documentation would be
+   appreciated but is not required. 
+2. Altered source versions must be plainly marked as such, and must not be
+   misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+

resources/SDL_GameControllerDB/README.md

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+##SDL_GameControllerDB
+
+[![Build Status](https://travis-ci.org/gabomdq/SDL_GameControllerDB.svg?branch=master)](https://travis-ci.org/gabomdq/SDL_GameControllerDB)
+
+A community source database of game controller mappings to be used with SDL2 Game Controller functionality.
+
+####Usage:
+
+Download gamecontrollerdb.txt, place it in your app's directory and load with:
+
+```
+SDL_GameControllerAddMappingsFromFile("gamecontrollerdb.txt");
+```
+
+####Creating new mappings:
+
+To create new mappings, you can use the controllermap utility provided with
+SDL2, or using Steam's Big Picture mode, configure your joystick and then 
+look in config/config.vdf in your Steam installation directory for the 
+SDL_GamepadBind entry.
+
+####Checking your mappings:
+You need to have python3 installed. Run
+
+```
+python3 check.py gamecontrollerdb.txt
+```
+
+####References:
+
+* [SDL2](http://www.libsdl.org)
+* [SDL_GameControllerAddMappingsFromFile](http://wiki.libsdl.org/SDL_GameControllerAddMappingsFromFile)