docs(): replace gitter by discord on contributing guide

CONTRIBUTING.md

@@ -27,7 +27,7 @@ Stack Overflow is a much better place to ask questions since:
 
 To save your and our time, we will systematically close all issues that are requests for general support and redirect people to Stack Overflow.
 
-If you would like to chat about the question in real-time, you can reach out via [our gitter channel][gitter].
+If you would like to chat about the question in real-time, you can reach out via [our discord channel][discord].
 
 ## <a name="issue"></a> Found a Bug?
 If you find a bug in the source code, you can help us by
@@ -244,7 +244,7 @@ changes to be accepted, the CLA must be signed. It's a quick process, we promise
 [corporate-cla]: http://code.google.com/legal/corporate-cla-v1.0.html
 [dev-doc]: https://github.com/nestjs/nest/blob/master/docs/DEVELOPER.md
 [github]: https://github.com/nestjs/nest
-[gitter]: https://gitter.im/nestjs/nest
+[discord]: https://discord.gg/nestjs
 [individual-cla]: http://code.google.com/legal/individual-cla-v1.0.html
 [js-style-guide]: https://google.github.io/styleguide/jsguide.html
 [jsfiddle]: http://jsfiddle.net