add SDL2 sources

thirdparty/SDL2-src/Xcode/SDL/pkg-support/resources/ReadMe.txt

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+The Simple DirectMedia Layer (SDL for short) is a cross-platform 
+library designed to make it easy to write multi-media software, 
+such as games and emulators.
+
+The Simple DirectMedia Layer library source code is available from: 
+http://www.libsdl.org/
+
+This library is distributed under the terms of the zlib license: 
+http://zlib.net/zlib_license.html
+
+
+This packages contains the SDL framework for OS X. 
+Conforming with Apple guidelines, this framework 
+contains both the SDL runtime component and development header files.
+
+
+To Install:
+Copy the SDL2.framework to /Library/Frameworks
+
+You may alternatively install it in <Your home directory>/Library/Frameworks 
+if your access privileges are not high enough. 
+
+
+Additional References:
+
+ - Screencast tutorials for getting started with OpenSceneGraph/Mac OS X are 
+ 	available at:
+	http://www.openscenegraph.org/projects/osg/wiki/Support/Tutorials/MacOSXTips
+	Though these are OpenSceneGraph centric, the same exact concepts apply to 
+	SDL, thus the videos are recommended for everybody getting started with
+	developing on Mac OS X. (You can skim over the PlugIns stuff since SDL
+	doesn't have any PlugIns to worry about.)