add test coverage badge

README.md

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 #gRPC-Go
 
-[![Build Status](https://travis-ci.org/grpc/grpc-go.svg)](https://travis-ci.org/grpc/grpc-go) [![GoDoc](https://godoc.org/google.golang.org/grpc?status.svg)](https://godoc.org/google.golang.org/grpc)
+[![Build Status](https://travis-ci.org/grpc/grpc-go.svg)](https://travis-ci.org/grpc/grpc-go) [![GoDoc](https://godoc.org/google.golang.org/grpc?status.svg)](https://godoc.org/google.golang.org/grpc) [![Coverage Status](https://coveralls.io/repos/grpc/grpc-go/badge.svg?branch=master&service=github)](https://coveralls.io/github/grpc/grpc-go?branch=master)
 
 The Go implementation of [gRPC](http://www.grpc.io/): A high performance, open source, general RPC framework that puts mobile and HTTP/2 first. For more information see the [gRPC Quick Start](http://www.grpc.io/docs/) guide.