docs: Update link to the new CNCF Slack channel (#1179)

Signed-off-by: terrytangyuan <terrytangyuan@gmail.com>

README.md

@@ -1,7 +1,7 @@
 # Argo Events - The Event-driven Workflow Automation Framework
 
 [![Go Report Card](https://goreportcard.com/badge/github.com/argoproj/argo-events)](https://goreportcard.com/report/github.com/argoproj/argo-events)
-[![slack](https://img.shields.io/badge/slack-argoproj-brightgreen.svg?logo=slack)](https://argoproj.github.io/community/join-slack)
+[![slack](https://img.shields.io/badge/slack-argo_events-brightgreen.svg?logo=slack)](https://cloud-native.slack.com/archives/C01TNKD6KL6)
 [![Build Status](https://travis-ci.org/argoproj/argo-events.svg?branch=master)](https://travis-ci.org/argoproj/argo-events)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/3832/badge)](https://bestpractices.coreinfrastructure.org/projects/3832)
 [![GoDoc](https://godoc.org/github.com/argoproj/argo-events?status.svg)](https://godoc.org/github.com/argoproj/argo-events/pkg/apis)