Add Slack Badge (#267)

README.md

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 # Go-Chassis  
-[![Build Status](https://travis-ci.org/ServiceComb/go-chassis.svg?branch=master)](https://travis-ci.org/ServiceComb/go-chassis)  [![Coverage Status](https://coveralls.io/repos/github/ServiceComb/go-chassis/badge.svg)](https://coveralls.io/github/ServiceComb/go-chassis) [![Go Report Card](https://goreportcard.com/badge/github.com/ServiceComb/go-chassis)](https://goreportcard.com/report/github.com/ServiceComb/go-chassis) [![GoDoc](https://godoc.org/github.com/ServiceComb/go-chassis?status.svg)](https://godoc.org/github.com/ServiceComb/go-chassis) [![HitCount](http://hits.dwyl.io/ServiceComb/go-chassis.svg)](http://hits.dwyl.io/ServiceComb/go-chassis)      
+[![Build Status](https://travis-ci.org/ServiceComb/go-chassis.svg?branch=master)](https://travis-ci.org/ServiceComb/go-chassis)  [![Coverage Status](https://coveralls.io/repos/github/ServiceComb/go-chassis/badge.svg)](https://coveralls.io/github/ServiceComb/go-chassis) [![Go Report Card](https://goreportcard.com/badge/github.com/ServiceComb/go-chassis)](https://goreportcard.com/report/github.com/ServiceComb/go-chassis) [![GoDoc](https://godoc.org/github.com/ServiceComb/go-chassis?status.svg)](https://godoc.org/github.com/ServiceComb/go-chassis) [![HitCount](http://hits.dwyl.io/ServiceComb/go-chassis.svg)](http://hits.dwyl.io/ServiceComb/go-chassis)  [![Join Slack](https://img.shields.io/badge/Join-Slack-orange.svg)](https://join.slack.com/t/go-chassis/shared_invite/enQtMzk0MzAyMjEzNzEyLTRjOWE3NzNmN2IzOGZhMzZkZDFjODM1MDc5ZWI0YjcxYjM1ODNkY2RkNmIxZDdlOWI3NmQ0MTg3NzBkNGExZGU)      
 
 Go-Chassis is a Software Development Kit(SDK) for rapid development of microservices in Go