CONTRIBUTING.md: Mention IRC/matrix before mailing list

Nowadays, the IRC and (since it is linked to it) matrix channels are
much faster ways to get into contact with us than the (very low traffic)
mailing list.  CONTRIBUTING should reflect that, since especially new
contributors might not know where to go to get a feeling for the people
working on the project—or worse, think we only communicate via pull
requests and issues!

CONTRIBUTING.md

@@ -34,9 +34,11 @@ Awesome!  Here are a few things to keep in mind:
     nontrivial changes to xmonad.  There are a couple of ways you can
     chat with us:
 
-    - Post a message to the [mailing list][ml].
+    - Join the [`#xmonad` IRC channel] on `irc.libera.chat` or the
+      official [matrix channel], which is linked to IRC.  This is the
+      preferred (and fastest!) way to get into contact with us.
 
-    - Join the `#xmonad` IRC channel on `irc.libera.chat`.
+    - Post a message to the [mailing list][ml].
 
   * [XMonad.Doc.Developing][xmonad-doc-developing] is a great
     resource to get an overview of xmonad.  Make sure to also check
@@ -184,3 +186,5 @@ each pull request contains just one commit.
 [x11]: https://github.com/xmonad/X11
 [ml]: https://mail.haskell.org/cgi-bin/mailman/listinfo/xmonad
 [xmonad-doc-developing]: https://xmonad.github.io/xmonad-docs/xmonad-contrib/XMonad-Doc-Developing.html
+[`#xmonad` IRC channel]: https://web.libera.chat/#xmonad
+[matrix channel]: https://matrix.to/#/#xmonad:matrix.org