auto-modes.el: document its purpose

diff --git a/auto-modes.el b/auto-modes.el
index 7d4c62f0d439489b03a82ceae531571d6c9458a8..59a41f3723b77e0b3210f209bce46df65895e9b1 100644 (file)
--- a/auto-modes.el
+++ b/auto-modes.el
@@ -1,3 +1,12 @@
+;;
+;; The foo-mode that are included with emacs usually contain their own
+;; default list of file extensions, and these are compiles into a
+;; default auto-mode-alist when emacs is built. Those, we don't have
+;; to worry about. The entries below thus fall into two categories:
+;;
+;; 1. Nonstandard extensions for upstream modes.
+;; 2. Extensions for modes that are not upstream.
+;;
 (setq auto-mode-alist
       (append
        (list