adding a slack shield (#140)

Author: domgreen

README.md

@@ -7,7 +7,7 @@
 [![codecov](https://codecov.io/gh/grpc-ecosystem/go-grpc-middleware/branch/master/graph/badge.svg)](https://codecov.io/gh/grpc-ecosystem/go-grpc-middleware)
 [![Apache 2.0 License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
 [![quality: production](https://img.shields.io/badge/quality-production-orange.svg)](#status)
-
+[![Slack](slack.png)](https://join.slack.com/t/improbable-eng/shared_invite/enQtMzQ1ODcyMzQ5MjM4LWY5ZWZmNGM2ODc5MmViNmQ3ZTA3ZTY3NzQwOTBlMTkzZmIxZTIxODk0OWU3YjZhNWVlNDU3MDlkZGViZjhkMjc)
 
 [gRPC Go](https://github.com/grpc/grpc-go) Middleware: interceptors, helpers, utilities.