Add Slack Workspace information and invite link.

This closes #31.

Signed-off-by: Aldrin Piri <aldrin@apache.org>
diff --git a/src/pages/html/mailing_lists.hbs b/src/pages/html/mailing_lists.hbs
index 64180b2..1a5da3c 100644
--- a/src/pages/html/mailing_lists.hbs
+++ b/src/pages/html/mailing_lists.hbs
@@ -120,6 +120,10 @@
         </p>
         <ul>
             <li>
+                Apache NiFi Slack Workspace: <a href="https://apachenifi.slack.com/">https://apachenifi.slack.com/</a> <br />
+                New users can join the workspace using the following <a href="https://join.slack.com/t/apachenifi/shared_invite/enQtNDI2NDMyMTY3MTA5LWJmZDI3MmM1ZmYyODQwZDYwM2MyMDY5ZjkyMDkxY2JmOGMyNmEzYTE0MTRkZTYwYzZlYTJkY2JhZTYyMzcyZGI">invite link</a>.
+            </li>
+            <li>
                 IRC:  #nifi on <a href="http://webchat.freenode.net/?channels=#nifi">irc.freenode.net</a>
             </li>
             <li>