fixing webcomponents.org demo

README.md

@@ -13,6 +13,7 @@ Example:
   <template>
     <script src="../webcomponentsjs/webcomponents-lite.js"></script>
     <link rel="import" href="paper-tree.html">
+    <next-code-block></next-code-block>
   </template>
 </custom-element-demo>
 ```