Manually add NonGNU elpa archive

This has been added by default in Emacs 28.1, so it's resonable to also include
it in my list.

init.el

@@ -46,7 +46,8 @@
       '(("melpa" . "https://melpa.org/packages/")
         ("gnu" . "https://elpa.gnu.org/packages/")
         ("melpa-stable" . "https://stable.melpa.org/packages/")
-        ("org" . "https://orgmode.org/elpa/")))
+        ("org" . "https://orgmode.org/elpa/")
+        ("nongnu" . "https://elpa.nongnu.org/nongnu/")))
 
 (when (< emacs-major-version 27)
   ;; Before Emacs 27.1, we had to do package initialization ourselves.  In Emacs