Added "get_unique_ID only works on iOS/Android" (#284)

tutorials/engine/encrypting_save_games.rst

@@ -54,4 +54,6 @@ some unique user identifier is needed, for example:
     f.store_var(game_state)
     f.close()
 
+Note that ``OS.get_unique_ID()`` only works on iOS and Android.
+
 This is all! Thanks for your cooperation, citizen.