add customizer link to docs homepage

docs/index.html

@@ -31,6 +31,9 @@ title: Bootstrap
     </div>
 
     <ul class="masthead-links">
+      <li>
+        <a href="./customize/" onclick="_gaq.push(['_trackEvent', 'Jumbotron actions', 'Jumbotron links', 'Customize']);">Customize</a>
+      </li>
       <li>
         <a href="http://github.com/twitter/bootstrap" onclick="_gaq.push(['_trackEvent', 'Jumbotron actions', 'Jumbotron links', 'GitHub project']);">GitHub project</a>
       </li>