ARROW-3114: [Website] Add information about user@ mailing list to website / Community page

This addresses
https://issues.apache.org/jira/browse/ARROW-3114

Author: Renato Marroquin <marenato@inf.ethz.ch>

Closes #2473 from renato2099/ARROW-3114 and squashes the following commits:

22d206ef <Renato Marroquin> ARROW-3114:  Add information about user@ mailing list to website / Community page
diff --git a/_includes/header.html b/_includes/header.html
index 9e4644c..b38d3e0 100644
--- a/_includes/header.html
+++ b/_includes/header.html
@@ -32,7 +32,8 @@
              aria-expanded="false">Community<span class="caret"></span>
           </a>
           <ul class="dropdown-menu">
-            <li><a href="http://mail-archives.apache.org/mod_mbox/arrow-dev/">Mailing List</a></li>
+            <li><a href="http://mail-archives.apache.org/mod_mbox/arrow-user/">User Mailing List</a></li>
+            <li><a href="http://mail-archives.apache.org/mod_mbox/arrow-dev/">Dev Mailing List</a></li>
             <li><a href="https://cwiki.apache.org/confluence/display/ARROW">Developer Wiki</a></li>
             <li><a href="{{ site.baseurl }}/committers/">Committers</a></li>
             <li><a href="{{ site.baseurl }}/powered_by/">Powered By</a></li>