Android: add comment to set render target to NULL when going to background (bug #4041)

docs/README-android.md

@@ -206,6 +206,9 @@ You should not use the SDL renderer API while the app going in background:
     after you read this message, GL context gets backed-up and you should not
     use the SDL renderer API.
 
+    When this event is received, you have to set the render target to NULL, if you're using it.
+    (eg call SDL_SetRenderTarget(renderer, NULL))
+
 - SDL_APP_DIDENTERFOREGROUND:
    GL context is restored, and the SDL renderer API is available (unless you
    receive SDL_RENDER_DEVICE_RESET).