[gtk] Update CONTRIBUTING.md

[Matthias Clasen <matthiasc src gnome org>]

commit b0fe4dec2cb7d022d0228955432b8ee7f7bdd880
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed May 6 15:08:36 2020 -0400

    Update CONTRIBUTING.md
    
    Point to discourse, rather than mailing lists.
    Based on a suggestion by sujiniku,
    
    https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/1763

 CONTRIBUTING.md | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 713c259190..474598fc81 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -22,12 +22,7 @@ Please, do not use the issue tracker for support questions. If you have
 questions on how to use GTK effectively, you can use:
 
  - the `#gtk` IRC channel on irc.gnome.org
- - the [gtk](https://mail.gnome.org/mailman/listinfo/gtk-list) mailing list,
-   for general questions on GTK
- - the [gtk-app-devel](https://mail.gnome.org/mailman/listinfo/gtk-app-devel-list)
-   mailing list, for questions on application development with GTK
- - the [gtk-devel](https://mail.gnome.org/mailman/listinfo/gtk-devel-list)
-   mailing list, for questions on developing GTK itself
+ - the [gtk tag on the GNOME Discourse instance](https://discourse.gnome.org/tag/gtk)
 
 You can also look at the GTK tag on [Stack
 Overflow](https://stackoverflow.com/questions/tagged/gtk).