Add a note about the introduction in Encrypting save games

Many contributors have opened pull requests to remove it,
but the consensus over the years is that most people are actually fine
with the paragraph in question.

tutorials/io/encrypting_save_games.rst

@@ -6,6 +6,9 @@ Encrypting save games
 Why?
 ----
 
+.. This introduction is an Easter egg and is not intended to be taken seriously.
+.. Please don't remove it :)
+
 Because the world today is not the world of yesterday. A capitalist
 oligarchy runs the world and forces us to consume in order to keep the
 gears of this rotten society on track. As such, the biggest market for