readme: Added slack badge with join link for go-grpc-prometheus channel.

Signed-off-by: Bartek Plotka <[email protected]>

Author: bwplotka

README.md

@@ -5,6 +5,7 @@
 [![GoDoc](http://img.shields.io/badge/GoDoc-Reference-blue.svg)](https://godoc.org/github.com/grpc-ecosystem/go-grpc-prometheus)
 [![SourceGraph](https://sourcegraph.com/github.com/grpc-ecosystem/go-grpc-prometheus/-/badge.svg)](https://sourcegraph.com/github.com/grpc-ecosystem/go-grpc-prometheus/?badge)
 [![codecov](https://codecov.io/gh/grpc-ecosystem/go-grpc-prometheus/branch/master/graph/badge.svg)](https://codecov.io/gh/grpc-ecosystem/go-grpc-prometheus)
+[![Slack](https://img.shields.io/badge/join%20slack-%23go--grpc--prometheus-brightgreen.svg)](https://join.slack.com/t/improbable-eng/shared_invite/enQtMzQ1ODcyMzQ5MjM4LWY5ZWZmNGM2ODc5MmViNmQ3ZTA3ZTY3NzQwOTBlMTkzZmIxZTIxODk0OWU3YjZhNWVlNDU3MDlkZGViZjhkMjc)
 [![Apache 2.0 License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
 
 [Prometheus](https://prometheus.io/) monitoring for your [gRPC Go](https://github.com/grpc/grpc-go) servers and clients.