add badge for gorepocard

README.md

@@ -1,6 +1,6 @@
 # go-isatty
 
-[![Build Status](https://travis-ci.org/mattn/go-isatty.svg?branch=master)](https://travis-ci.org/mattn/go-isatty) [![Coverage Status](https://coveralls.io/repos/github/mattn/go-isatty/badge.svg?branch=master)](https://coveralls.io/github/mattn/go-isatty?branch=master)
+[![Build Status](https://travis-ci.org/mattn/go-isatty.svg?branch=master)](https://travis-ci.org/mattn/go-isatty) [![Coverage Status](https://coveralls.io/repos/github/mattn/go-isatty/badge.svg?branch=master)](https://coveralls.io/github/mattn/go-isatty?branch=master) [![Go Report Card](https://goreportcard.com/badge/mattn/go-isatty)](https://goreportcard.com/report/mattn/go-isatty)
 
 isatty for golang