Added physics 2d option

docs/en/manuals/app-manifest.md

@@ -20,6 +20,9 @@ In `game.project`, assign the manifest to `Native Extensions` -> `App Manifest`.
 
 Control which physics engine to use, or select None to exclude physics completely.
 
+## Physics 2d
+
+Select which version of Box2D to use.
 
 ## Rig + Model