Add GitHub star button to header.

layouts/header.html

@@ -72,6 +72,9 @@
             <li><a href="/community/">Community</a></li>
             <li><a href="/blog/">Blog</a></li>
             <li><a href="https://github.com/prometheus"><i class="fa fa-github"></i> Github</a></li>
+            <li>
+              <iframe src="https://ghbtns.com/github-btn.html?user=prometheus&repo=prometheus&type=star&count=true" class="github-stars" frameborder="0" scrolling="0"></iframe>
+            </li>
           </ul>
         </div>
       </div>

static/docs.css

@@ -19,6 +19,12 @@ body {
   margin-right: 3px;
 }
 
+.navbar .github-stars {
+  margin: 15px 0 0 15px;
+  width: 100px;
+  height: 20px;
+}
+
 .jumbotron {
   background-color: #e6522c;
   background-image: url("/assets/jumbotron-background.png");