Add discourse in the community dropdown (#55)

Author: ian-r-rose

index.html

@@ -71,6 +71,7 @@
               <div class="dropdown-menu" aria-labelledby="navbarDropdownMenuLink">
                 <a class="dropdown-item" href="https://docs.dask.org/en/latest/support.html">Ask for Help</a>
                 <a class="dropdown-item" href="https://github.com/dask">Github</a>
+                <a class="dropdown-item" href="https://dask.discourse.group">Discourse Help Forum</a>
                 <a class="dropdown-item" href="https://stackoverflow.com/questions/tagged/dask">Stack Overflow</a>
                 <a class="dropdown-item" href="https://twitter.com/dask_dev">Twitter</a>
                 <a class="dropdown-item" href="https://blog.dask.org/"> Developer Blog </a>