website: mention new mailing list for discussions

website/contact/index.html

@@ -94,5 +94,12 @@ a {
   think before you ask :-).
 </p>
 
+<h2>Mailing list</h2>
+
+<p>
+  If you prefer to discuss via mail rather than on IRC, you can also post to our
+  mailing list. Subscribe by sending a mail to <code>i3-discuss-subscribe@i3.zekjur.net</code>
+</p>
+
 </body>
 </html>