Merge pull request #181 from domgreen/slack_shield

adding slack shield

README.md

@@ -7,6 +7,7 @@
 [![GoDoc](http://img.shields.io/badge/GoDoc-Reference-blue.svg)](https://godoc.org/github.com/improbable-eng/grpc-web/go/grpcweb) 
 [![Apache 2.0 License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
 [![quality: alpha](https://img.shields.io/badge/quality-alpha-orange.svg)](#status)
+[![Slack](slack.png)](https://join.slack.com/t/improbable-eng/shared_invite/enQtMzQ1ODcyMzQ5MjM4LWY5ZWZmNGM2ODc5MmViNmQ3ZTA3ZTY3NzQwOTBlMTkzZmIxZTIxODk0OWU3YjZhNWVlNDU3MDlkZGViZjhkMjc)
 
 [gRPC](http://www.grpc.io/) is a modern, [HTTP2](https://hpbn.co/http2/)-based protocol, that provides RPC semantics using the strongly-typed *binary* data format of [protocol buffers](https://developers.google.com/protocol-buffers/docs/overview) across multiple languages (C++, C#, Golang, Java, Python, NodeJS, ObjectiveC, etc.)