Update docs from gitter to slack for communication (#582)

Author: MrAlias

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 +3,7 @@
 [![build_and_test](https://github.com/open-telemetry/opentelemetry-go-contrib/workflows/build_and_test/badge.svg)](https://github.com/open-telemetry/opentelemetry-go-contrib/actions?query=workflow%3Abuild_and_test+branch%3Amain)
 [![Docs](https://godoc.org/go.opentelemetry.io/contrib?status.svg)](https://pkg.go.dev/go.opentelemetry.io/contrib)
 [![Go Report Card](https://goreportcard.com/badge/go.opentelemetry.io/contrib)](https://goreportcard.com/report/go.opentelemetry.io/contrib)
-[![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)
 
 Collection of 3rd-party instrumentation and exporters for [OpenTelemetry-Go](https://github.com/open-telemetry/opentelemetry-go).