Document external maintenance of some packages

Document external maintenance of some packages

Document the fact that some packages in Emacs are externally
maintained, and specifically document Org Mode's external maintenance.

For more context, see the thread that starts here:

  https://lists.gnu.org/archive/html/emacs-devel/2021-12/msg00366.html
  From: Karl Fogel
  To: Emacs Devel
  Subject: Improving documentation of Org Mode integration into Emacs.
  Date: Sat, 04 Dec 2021 15:14:59 -0600
  Message-ID: <87zgpgax7w.fsf@red-bean.com>