Adding link to Dafny Zulip channel (#46)

Co-authored-by: Fabio Madge <[email protected]>

index.html

@@ -30,6 +30,7 @@ <h1>The Dafny Programming and Verification Language</h1>
 <ul>
 <li><a href="./latest/Installation">Install</a>
      (or just use the VS Code <a href="https://marketplace.visualstudio.com/items?itemName=dafny-lang.ide-vscode">extension</a>)</li>
+<li><a href="https://dafny.zulipchat.com/" target="_blank" rel="noopener noreferrer">Zulip channel</a> to ask questions about Dafny</li>
 <li><a href="./latest/DafnyRef/DafnyRef">Reference Manual and User Guide</a></li>
 <li><a href="./latest/toc">Resources for Users</a></li>
 <li><a href="./blog">Blog</a></li>