change gitter for slack

README.md

@@ -13,4 +13,4 @@ nf-core/test-datasets comes with documentation in the `docs/` directory:
 
 ## Support
 
-For further information or help, don't hesitate to get in touch on our [Gitter channel](https://gitter.im/nf-core/Lobby)
+For further information or help, don't hesitate to get in touch on our [Slack](https://nfcore.slack.com) or [Click here for an invite](https://nf-core-invite.herokuapp.com/)