Link to castholm's repo for SDL3 support

Author: jayschwa

README.md

@@ -1,3 +1,3 @@
-This is a fork of [SDL](https://www.libsdl.org/), packaged for Zig. Unnecessary
-files have been deleted, and the build system has been replaced with
-`build.zig`.
+This is a fork of [SDL](https://www.libsdl.org/), packaged for [Zig](https://ziglang.org).
+Unnecessary files have been deleted, and the build system has been replaced with `build.zig`.
+The package provides version 2 of SDL. For version 3, consider https://github.com/castholm/SDL.