Document contact options in library commentary

I used to believe that it was enough that the library header
contains

  ;; Author: Jonas Bernoulli <[email protected]>
  ;; Homepage: https://github.com/magit/transient

and https://elpa.gnu.org/packages/transient.html contains

  Maintainer
  Jonas Bernoulli <[email protected]>
  Website
  https://github.com/magit/transient

and the output of M-x describe-package RET transient RET contains

    Website: https://github.com/magit/transient
 Maintainer: Jonas Bernoulli <[email protected]>
     Author: Jonas Bernoulli <[email protected]>

to imply that either one or the other can be used to contact me,
the maintainer, about Transient; but the consensus on emacs-devel
seems to be that this information has to be spelled out in the
library commentary, so that's what I do now.

(Also note that the email address contains the substring "transient",
which implies that it is intended for inquiries about Transient.)

Author: tarsius

lisp/transient.el

@@ -36,6 +36,12 @@
 ;; in Magit.  It is distributed as a separate package, so that it can be
 ;; used to implement similar menus in other packages.
 
+;; You can contact the maintainer of this package by sending an email to
+;; <[email protected]>, or you can use the public issue
+;; tracker at <https://github.com/magit/transient>.  The latter can also
+;; be accessed using the `forge' package, which lets you avoid the sadly
+;; non-free javascript on that website.
+
 ;;; Code:
 
 (defconst transient-version "0.12.0")