Add step re. setting main scene

My Windows executable would not run until I figured this out.

getting_started/step_by_step/exporting.rst

@@ -119,6 +119,13 @@ changed:
         emit_signal("hit")
         $CollisionShape2D.set_deferred("disabled", true)
 
+Setting a main scene
+--------------------
+
+The main scene is the one that your game will start in. In *Project -> Project 
+Settings -> Application -> Run*, set *Main Scene* to "Main.tscn" by clicking 
+the folder icon and selecting it.
+
 Export templates
 ----------------