Update CONTRIBUTING.md (phoenixframework#3051)

Updated mailing list link noting it's recent depreciation in favor the Phoenix Forum.

CONTRIBUTING.md

@@ -13,7 +13,8 @@ Use the issues tracker for:
 
 Please **do not** use the issue tracker for personal support requests nor feature requests. Support requests should be sent to:
 
-* [the phoenix-talk mailing list](http://groups.google.com/group/phoenix-talk)
+* [the phoenix-talk mailing list](http://groups.google.com/group/phoenix-talk) (closed in favor of forum, but archive is still online)
+* [The Phoenix subforum on the Elixir forum](https://elixirforum.com/c/phoenix-forum)
 * **[#elixir-lang](irc://chat.freenode.net/elixir-lang)** IRC channel on [chat.freenode.net](http://www.freenode.net/)
 
 Development issues can be discussed on [the phoenix-core mailing list](http://groups.google.com/group/phoenix-core).