Add a handy mailto: link to file a new bug report

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

index.html

@@ -122,7 +122,10 @@ layout: default
     </p>
     <ul>
       <li><a href="https://salsa.debian.org/reproducible-builds/diffoscope">Git repository</a></li>
-      <li><a href="https://bugs.debian.org/src:diffoscope">Bugs and feature requests</a></li>
+      <li>
+        <a href="https://bugs.debian.org/src:diffoscope">Bugs and feature requests</a>
+        <ul><li><a href="mailto:submit@bugs.debian.org?subject=diffoscope%3A%20bug%20report%20title&body=Package%3A%20diffoscope%0AVersion%3A%20FIXME%0A%0ADescribe%20here%20your%20problem">File a new bug!</a></li></ul>
+      </li>
       <li><a href="https://lists.reproducible-builds.org/listinfo/diffoscope">Users and developers mailing-list</a></li>
       <li><code>#reproducible-builds</code> and/or <code>#debian-reproducible</code> on <a href="https://oftc.net/">OFTC</a></li>
     </ul>