docs(contributing): replace gitter by discord

Author: Tony133

CONTRIBUTING.md

@@ -18,7 +18,7 @@ We would love for you to contribute to Nest and help make it even better than it
 
 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](https://gitter.im/nestjs/nest-cli) channel.
+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?
 
@@ -217,7 +217,7 @@ changes to be accepted, the CLA must be signed. It's a quick process, we promise
 [commit-message-format]: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#
 [corporate-cla]: http://code.google.com/legal/corporate-cla-v1.0.html
 [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