Add SDL2 sources

SDL2-2.0.12/INSTALL.txt

@@ -0,0 +1,40 @@
+
+To compile and install SDL:
+
+    1.  Windows with Visual Studio:
+        * Read VisualC.html
+
+        Windows with gcc, either native or cross-compiling:
+        * Read the FAQ at https://wiki.libsdl.org/moin.fcg/FAQWindows
+        * Run './configure; make; make install'
+
+        Mac OS X with Xcode:
+        * Read docs/README-macosx.md
+
+        Mac OS X from the command line:
+        * Run './configure; make; make install'
+
+        Linux and other UNIX systems:
+        * Run './configure; make; make install'
+
+        Android:
+        * Read docs/README-android.md
+
+        iOS:
+        * Read docs/README-ios.md
+
+        Using Cmake:
+        * Read docs/README-cmake.md
+
+    2.  Look at the example programs in ./test, and check out the online
+        documentation at https://wiki.libsdl.org/
+
+    3.  Join the SDL developer mailing list by sending E-mail to
+    	sdl-request@libsdl.org
+        and put "subscribe" in the subject of the message.
+
+        Or alternatively you can use the web interface:
+            https://www.libsdl.org/mailing-list.php
+
+That's it!
+Sam Lantinga <slouken@libsdl.org>