Add PLATFORM_SDL

raylib-GLFW-dependency.md

@@ -9,6 +9,7 @@ GLFW is *not* used on the following platforms, where custom implementations are
 
   - `PLATFORM_ANDROID`: Uses the `native_app_glue` Android NDK module.
   - `PLATFORM_RPI`: (native, no desktop) Uses `EGL`, `evdev` and standard system libraries directly.
+  - `PLATFORM_SDL`: Uses the native SDL functions.
 
 If you are interested in replacing GLFW with a custom, platform-specific implementation, the functions currently used by raylib (as of raylib 4.0) are detailed below: