Add PromCon link to landing page jumbotron.

layouts/jumbotron.html

@@ -4,7 +4,10 @@
   <div class="container">
     <h1><img src="/assets/prometheus_logo.svg" alt="Prometheus logo"> Prometheus</h1>
     <p class="subtitle">An open-source service monitoring system and time series database.</p>
-    <p><a class="btn btn-default btn-lg" href="/docs/introduction/getting_started/" role="button">Get Started</a></p>
+    <p>
+      <a class="btn btn-default btn-lg" href="/docs/introduction/getting_started/" role="button">Get Started</a>
+      <a class="btn btn-default btn-lg" href="https://promcon.io/" role="button">PromCon 2016</a>
+    </p>
   </div>
 </div>