add goreportcard to readme.md

README.md

@@ -1,6 +1,7 @@
 [![GoDoc](https://godoc.org/github.com/alexflint/go-arg?status.svg)](https://godoc.org/github.com/alexflint/go-arg)
 [![Build Status](https://travis-ci.org/alexflint/go-arg.svg?branch=master)](https://travis-ci.org/alexflint/go-arg)
 [![Coverage Status](https://coveralls.io/repos/alexflint/go-arg/badge.svg?branch=master&service=github)](https://coveralls.io/github/alexflint/go-arg?branch=master)
+[![Report Card](https://goreportcard.com/badge/github.com/alexflint/go-arg)](https://goreportcard.com/badge/github.com/alexflint/go-arg)
 
 ## Structured argument parsing for Go