Add documentation for dancing dragon extensions

doc/extension_doc.md

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+# mkxp extenions for Dancing Dragon Games / Symphony of War
+
+## Graphics.resize_window(width, height, recenter)
+width: Integer  
+height: Integer  
+recenter: Boolean, false by default  
+returns: nil  
+
+Resizes the game window to width x height. If `recenter` is **true**, also center the window on the current screen.
+
+## Bitmap.write_to_png(filename)
+filename: String  
+returns: self  
+
+Writes the contents of the bitmap to `filename`, in PNG format.
+
+## Input.scroll_v()
+returns: Integer  
+
+Returns the cumulative amount of scroll events (negative if down, positive if up) inbetween the current and last `Input.update` call.
+
+## Arbitrary key states
+Use `MKXP.raw_key_states` to get the current byte array of keystates, then call `#getbyte(scancode)` with `scancode` being one of the constants defined in `SDL_scancode_map.rb`. **0** means the key is released, **1** that it is pressed.