添加vue2 changes链接

Author: dingyiming

themes/vue/layout/index.ejs

@@ -5,6 +5,7 @@
       <li><a href="/api/" class="nav-link">API</a></li>
       <li><a href="/examples/" class="nav-link">示例</a></li>
       <li><a href="https://github.com/vuejs/vue/releases" target="_blank" class="nav-link">更新</a></li>
+      <li><a href="https://github.com/vuejs/vue/issues/2873" target="_blank" class="nav-link">Vue2 changes</a></li>
       <li><a href="/blog" class="nav-link">博客</a></li>
       <%- partial('partials/community_dropdown') %>
     </ul>

themes/vue/layout/partials/main_menu.ejs

@@ -7,5 +7,7 @@
 <li><a href="/api/" class="nav-link<%- page.path.match(/api/) ? ' current' : '' %>">API</a></li>
 <li><a href="/examples/" class="nav-link<%- page.path.match(/examples/) ? ' current' : '' %>">示例</a></li>
 <li><a href="https://github.com/vuejs/vue/releases" target="_blank">更新</a></li>
+<li><a href="https://github.com/vuejs/vue/issues/2873" target="_blank">Vue2 changes</a></li>
+
 <li><a href="/blog/" class="nav-link<%- page.path.match(/blog/) ? ' current' : '' %>">博客</a></li>
 <%- partial('partials/community_dropdown') %>