Merge pull request #4332 from bjornmagnusson/contribution-guide-slack

CONTRIBUTING.md: Corrected slack channel/s in support channels

CONTRIBUTING.md

@@ -31,7 +31,9 @@ apply to [third_party](third_party/) and [vendor](vendor/).
 Whether you are a user or contributor, official support channels include:
 
 - GitHub [issues](https://github.com/kubernetes/helm/issues/new)
-- Slack: #Helm room in the [Kubernetes Slack](http://slack.kubernetes.io/)
+- Slack [Kubernetes Slack](http://slack.kubernetes.io/):
+  - User: #helm-users
+  - Contributor: #helm-dev
 
 Before opening a new issue or submitting a new pull request, it's helpful to search the project - it's likely that another user has already reported the issue you're facing, or it's a known issue that we're already aware of.