Add github link to help dialog

index.html

@@ -105,6 +105,7 @@
           <td>toggles this help modal</td>
         </tr>
       </table>
+      <a href="https://github.com/thallada/proximity-structures">Fork on Github</a>
     </div>
     <div id="controls" class="panel" style="display: none;">
       <button type="button" id="close-controls">x</button>