Added links to the Slack channel in website (apache#598)

site/_includes/navbar.html

@@ -75,7 +75,7 @@
         </a>
         <div class="dropdown-menu" aria-labelledby="versionsDropdown">
           <h3 class="dropdown-header">Get in touch</h3>
-          <a class="dropdown-item" href="{{ site.baseurl }}mailing-lists">Mailing Lists</a>
+          <a class="dropdown-item" href="{{ site.baseurl }}contact">Contact</a>
           <a class="dropdown-item" href="https://twitter.com/Apache_Pulsar">Twitter</a>
           <a class="dropdown-item" href="https://github.com/apache/incubator-pulsar/wiki">Wiki</a>
           <a class="dropdown-item" href="https://github.com/apache/incubator-pulsar/issues">Issue tracking</a>

site/mailing-lists.md → site/contact.md

@@ -1,14 +1,24 @@
 ---
-title: Mailing lists
+title: Contact
 layout: content
 ---
 
 
 There are many ways to get help from the Apache Pulsar community. The mailing lists are the primary place where all Pulsar committers are present. Bugs and feature requests can either be discussed
 on the dev mailing list or by opening an issue on [GitHub]({{ site.pulsar_repo }}).
 
+## Mailing Lists
+
 Name | Scope |  |  |  |
 :----|:------|:-|:-|:-|
 **[email protected]**   | User-related discussions        |  [Subscribe](mailto:[email protected])  |  [Unsubscribe](mailto:[email protected])  |  [Archives](http://mail-archives.apache.org/mod_mbox/incubator-pulsar-users/)  |
 **[email protected]**     | Development-related discussions |   [Subscribe](mailto:[email protected])   |   [Unsubscribe](mailto:[email protected])   |   [Archives](http://mail-archives.apache.org/mod_mbox/incubator-pulsar-dev/)   |
 **[email protected]** | All commits to the Pulsar repository   | [Subscribe](mailto:[email protected]) | [Unsubscribe](mailto:[email protected]) | [Archives](http://mail-archives.apache.org/mod_mbox/incubator-pulsar-commits/) |
+
+## Slack
+
+There is a Pulsar slack channel that is used for informal discussions for devs and users.
+
+The Slack instance is at https://apache-pulsar.slack.com/
+
+You can self-register at https://apache-pulsar.herokuapp.com/