Add forums link under Resources dropdown (#64)

* Add forums link under Resources dropdown

This commits adds PyTorch Forums link to resources dropdown

Author: Bgreen23

_includes/main_menu.html

@@ -54,6 +54,10 @@
             <span class=dropdown-title>About</span>
             <p>Learn about PyTorch’s features and capabilities</p>
           </a>
+          <a class="nav-dropdown-item" href="{{ site.external_urls.discuss }}" target="_blank">
+            <span class=dropdown-title>Forums</span>
+            <p>A place to discuss PyTorch code, issues, install, research</p>
+          </a>
         </div>
       </div>
     </li>