add performance test link

Author: jemygraw

demo/views/index.html

@@ -28,6 +28,7 @@
           <ul class="nav navbar-nav">
             <li class="active"><a href="#">上传示例</a></li>
             <li><a href="http://developer.qiniu.com/code/v6/sdk/javascript.html">SDK 文档</a></li>
+            <li><a href="/performance">上传速度检测</a></li>
           </ul>
         </div>
       </div>