add link to legacy docs

Author: yyx990803

themes/vue/layout/partials/sidebar.ejs

@@ -8,6 +8,7 @@
             <% site.pages.find({type: type}).sort('order').each(function (p) { %>
                 <li><a href="/<%- p.path %>" class="sidebar-link<%- page.title === p.title ? ' current' : '' %>"><%- p.title %></a></li>
             <% }) %>
+            <li><a href="http://legacy.vuejs.org">Looking for 0.10 docs?</a></li>
             <li style="margin:10px 0 3px">
               <script data-gittip-username="yyx990803"
                 data-gittip-widget="button"