add mathjax plugin to list

Author: onukura

index.html

@@ -185,6 +185,12 @@ <h2>
           </h2>
           <p>A plugin for rendering a R Markdown files.</p>
       </li>
+      <li>
+          <h2>
+              <a class="post-link" href="https://github.com/onukura/gitbucket-mathjax-plugin"><span class="fa fa-superscript"></span>MathJax plugin</a>
+          </h2>
+          <p>A plugin for rendering a markdown files with MathJax.</p>
+      </li>
 </ul>
     <p class="separator"/>
     <p>Your plugin is not listed here? You want to move your plugin under the organization? Please open a <a href="https://github.com/gitbucket-plugins/gitbucket-plugins.github.io/issues">ticket/issue</a> to reference your demand.</p>