add documentation link to ReadTheDocs

index.html

@@ -62,7 +62,7 @@
               <a class="nav-link" href="">Screenshots</a>
             </li>
             <li class="nav-item">
-              <a class="nav-link" href="">Documentations</a>
+              <a class="nav-link" href="http://vnote.readthedocs.io">Documentations</a>
             </li>
           </ul>
         </div>