Update docs from gitter to slack for communication (#1554)

Co-authored-by: Anthony Mirabella <a9@aneurysm9.com>

CONTRIBUTING.md

@@ -9,7 +9,7 @@ See the [public meeting
 notes](https://docs.google.com/document/d/1A63zSWX0x2CyCK_LoNhmQC4rqhLpYXJzXbEPDUQ2n6w/edit#heading=h.9tngw7jdwd6b)
 for a summary description of past meetings. To request edit access,
 join the meeting or get in touch on
-[Gitter](https://gitter.im/open-telemetry/opentelemetry-go).
+[Slack](https://cloud-native.slack.com/archives/C01NPAXACKT).
 
 ## Development
 

README.md

@@ -3,7 +3,8 @@
 [![CI](https://github.com/open-telemetry/opentelemetry-go/workflows/ci/badge.svg)](https://github.com/open-telemetry/opentelemetry-go/actions?query=workflow%3Aci+branch%3Amain)
 [![PkgGoDev](https://pkg.go.dev/badge/go.opentelemetry.io/otel)](https://pkg.go.dev/go.opentelemetry.io/otel)
 [![Go Report Card](https://goreportcard.com/badge/go.opentelemetry.io/otel)](https://goreportcard.com/report/go.opentelemetry.io/otel)
-[![Gitter](https://badges.gitter.im/open-telemetry/opentelemetry-go.svg)](https://gitter.im/open-telemetry/opentelemetry-go?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+[![Slack](https://img.shields.io/badge/slack-@cncf/otel--go-brightgreen.svg?logo=slack)](https://cloud-native.slack.com/archives/C01NPAXACKT)
+
 
 The Go [OpenTelemetry](https://opentelemetry.io/) implementation.