Add Slack registration link to footer.

src/site/_includes/footer.html

 
   <footer class="footer container-full hidden-print">
     <div class="container">
       <div class="row">
         <div class="col-md-5">
           <h3>Scalable, productive app development</h3>
           <p>Dart is an <a href="https://github.com/dart-lang/sdk">open-source project</a> with contributors from Google and elsewhere.</p>
           <p class="sm">Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 3.0 License, and code samples are licensed under the BSD License.</p>
           <p><a href="/tos.html">Terms</a> | <a href="http://www.google.com/intl/en/policies/privacy/">Privacy</a></p>
         </div>
@@ skipping 19 matching lines @@
             <li><a href="https://github.com/dart-lang/dartlang.org">www.dartlang.org repo</a></li>
           </ul>
         </div>
         <div class="col-md-2">
           <h4>Community</h4>
           <ul>
             <li><a href="/support/">Mailing lists</a></li>
             <li><a href="https://plus.google.com/communities/114566943291919232850">G+ community</a></li>
             <li><a href="https://plus.google.com/+dartlang/posts">G+ announcement group</a></li>
             <li><a href="http://stackoverflow.com/questions/tagged/dart">Stack Overflow</a></li>
-            <li><a href="https://dartlang.slack.com/">Slack</a></li>
+            <li><a href="https://dartlang.slack.com/">Slack</a>
+              (<a href="https://dartlang-slack.herokuapp.com/">register</a>)</li>
           </ul>
         </div>
       </div>
     </div>
   </footer> <!-- End footer -->
 
 {% comment %}Put all JS here.{% endcomment %}
 
 {% bundle %}
 - /js/jquery.js
@@ skipping 16 matching lines @@
 {% endbundle %}
 
 {% if page.header and page.header.css %}{% for css in page.header.css %}<link href="{{ css }}" rel="stylesheet" type="text/css">{% endfor %}{% endif %}
 
 {% for js in page.js %}
 <script {% if js.defer %}defer{% endif %} src="{{ js.url }}"></script>
 {% endfor %}
 
 </body>
 </html>