Clarify that -announce is _not_ mirrored to prometheus-users

Signed-off-by: Bjoern Rabenstein <bjoern@rabenste.in>

content/community.html

@@ -16,8 +16,8 @@ title: Community
     <p>
       <strong>User mailing lists:</strong>
       <ul>
-        <li><a href="https://groups.google.com/forum/#!forum/prometheus-announce">prometheus-announce</a> – for announcements like new releases</li>
-        <li><a href="https://groups.google.com/forum/#!forum/prometheus-users">prometheus-users</a> – for discussions around Prometheus usage and community support</li>
+        <li><a href="https://groups.google.com/forum/#!forum/prometheus-announce">prometheus-announce</a> – low-traffic list for announcements like new releases.</li>
+        <li><a href="https://groups.google.com/forum/#!forum/prometheus-users">prometheus-users</a> – for discussions around Prometheus usage and community support. Announcements are <em>not</em> generally mirrored from <a href="https://groups.google.com/forum/#!forum/prometheus-announce">prometheus-announce</a>.</li>
       </ul>
     </p>
     <p>