Add Twitter account to Community page.

content/community.html

@@ -9,6 +9,10 @@
     <strong>Mailing list:</strong>
     <a href="https://groups.google.com/forum/#!forum/prometheus-developers">prometheus-developers</a> Google Group
   </p>
+  <p>
+    <strong>Twitter:</strong>
+    <a href="https://twitter.com/PrometheusIO">PrometheusIO</a>
+  </p>
   <p>
     <strong>IRC:</strong> <code>#prometheus</code> on <a href="http://freenode.net/">irc.freenode.net</a>
   </p>