Make purpose of quarkus-dev mailing list more clear

The current description makes it sound like a general-purpose user forum, which it is now.

Author: holly-cummins

.github/ISSUE_TEMPLATE/config.yml

@@ -11,4 +11,4 @@ contact_links:
     about: If you prefer live chat with the developers, we have a Zulip chat where we all hang out.
   - name: Quarkus Development mailing list 
     url: https://groups.google.com/forum/#!forum/quarkus-dev
-    about: You can also ask questions on our mailing list.
+    about: Mailing list for discussing development of Quarkus itself, and development of Quarkus extensions