contrib/lisp/ox-rss.el: Mention that Emacs 24.1 at least is required

contrib/lisp/ox-rss.el

@@ -25,6 +25,8 @@
 ;; This library implements a RSS 2.0 back-end for Org exporter, based on
 ;; the `html' back-end.
 ;;
+;; It requires Emacs 24.1 at least.
+;;
 ;; It provides two commands for export, depending on the desired output:
 ;; `org-rss-export-as-rss' (temporary buffer) and `org-rss-export-to-rss'
 ;; (as a ".xml" file).