Updated Overlay image configuration (markdown)

Overlay-image-configuration.md

@@ -46,6 +46,14 @@ The default (stretch to full screen) could be described as such:
 
     overlay0_rect = "0.0,0.0,1.0,1.0"
 
+### Full-screen overlays
+
+By default, overlays will be stretched out to fill game viewport. However, in some cases
+the aspect ratio of the game causes there to remain large black borders around the game image.
+It is possible to stretch the overlay to full screen (instead of viewport) by specifying this option:
+
+    overlay0_full_screen = true
+
 ### Coordinate descriptors
 
 We must also describe where on the overlay image buttons can be found for each overlay.