init: add comment on version below 24.5

Signed-off-by: Luigi Santivetti <luigi.santivetti@gmail.com>
diff --git a/init/init.el b/init/init.el
index 2af778e..e33b1c4 100644
--- a/init/init.el
+++ b/init/init.el
@@ -38,6 +38,7 @@
 
 (add-to-list 'package-archives
 	     '("melpa" . "https://melpa.org/packages/") t)
+;; If running below the recommended Stretch version
 (when (< emacs-major-version 24)
   (add-to-list 'package-archives
 	       '("gnu" . "https://elpa.gnu.org/packages/")))