added slack badge

Author: Christian Strappazzon

index.html

@@ -98,7 +98,7 @@ <h4><i class="fa fa-trello fa-5"></i>
                 <div class="h-caption">
                     <h4>
                         <i class="fa fa-slack fa-5"></i>
-                        <a href="https://pythonmilano.slack.com/" class="btn-social btn-outline">Slack</a>
+                        <script async defer src="https://pythonmilano.herokuapp.com/slackin.js?large"></script>
                     </h4>
                 </div>
                 <div class="h-body text-center">