Add explanation of how to make cookie.el

compatible with strfile(1) to comment.

lisp/play/cookie1.el

@@ -44,6 +44,10 @@
 ;; formats could be supported by adding alternates to the regexp
 ;; `cookie-delimiter'.
 ;;
+;; strfile(1) is the program used to compile the files for fortune(6).
+;; In order to achieve total compatibility with strfile(1), cookie files
+;; should start with two consecutive delimiters (and no comment).
+;;
 ;; This code derives from Steve Strassman's 1987 spook.el package, but
 ;; has been generalized so that it supports multiple simultaneous
 ;; cookie databases and fortune files.  It is intended to be called