Add Slack Node.js Community (#1777)

* Add Slack Node.js Community

* add registration to slack

* remove Gitter

Author: aymen94

README.md

@@ -40,7 +40,8 @@ If you would prefer to get help via live chat rather than the issue tracker in
 this repository, you can try:
 
 * [Freenode #node.js channel](https://webchat.freenode.net/?channels=node.js&uio=d4)
-* [Gitter Node.js room](https://gitter.im/nodejs/node)
-
+* [Node.js Slack Community](https://node-js.slack.com/)
+  * To register: [nodeslackers.com](http://www.nodeslackers.com/)
+  
 ## Participation
 Want to help others with issues? You can start simply, by answering open questions.