Add note on pressing F6 for current scene vs. F5 for default scene

When following the tutorial instructions I reached the point of running the scene to test the button that starts and stops movement of the Godot icon.  After following the instructions explicitly, you will have the node_2d scene selected, but you are currently instructed to "run the scene".  If you choose to do so by pressing F5 or the play button in the UI, the default scene will run, which does not contain the button.

getting_started/step_by_step/signals.rst

@@ -254,6 +254,11 @@ Your complete ``sprite_2d.gd`` code should look like the following.
 
 Run the scene now and click the button to see the sprite start and stop.
 
+.. note:: If you try to run your scene and you don't see the button you created, you may have
+          run the default scene by pressing :kbd:`F5` (:kbd:`Cmd + B` on macOS) rather than the 
+          current scene by pressing :kbd:`F6` (:kbd:`Cmd + R` on macOS).  Make sure you have the
+          ``node_2d`` scene selected and then press :kbd:`F6` (:kbd:`Cmd + R` on macOS).
+
 Connecting a signal via code
 ----------------------------