Removed the runtime README

The main README is suitable for inclusion in redistributable archives and we'll have custom installation instructions for each platform.

README-SDL.txt

@@ -1,13 +0,0 @@
-
-Please distribute this file with the SDL runtime environment:
-
-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:
-https://www.libsdl.org/
-
-This library is distributed under the terms of the zlib license:
-http://www.zlib.net/zlib_license.html
-