add usage instructions

README.md

@@ -3,4 +3,12 @@ This is an application_surface wrapper extension for Game Maker 8.2.
 
 It automatically manages a surface, providing a post-draw callback entrypoint for composing the screen with it.
 
-Requires [Core](https://github.com/omicronrex/gm82core) version 1.3.2 or newer.
+Requires [Core](https://github.com/omicronrex/gm82core) version 1.3.2 or newer.
+
+## Usage
+1. Install Core and AppSurf, and add both to your project.
+2. Create a script that will be used as a screen composing callback.
+3. At game start, surface_init(...) with your callback script's id.
+4. Write screen composition code in the script. application_surface contains the surface handle, and the projection is set to the size of the surface.
+5. If need be, you can resize the application_surface.
+6. When using other surfaces during the draw event, make sure to utilize surface_disengage() instead of surface_reset_target() to properly reset the draw stack. Using surfaces during other events is fine.