Add GoReportCard badge to README

Author: gliptak

README.md

@@ -1,6 +1,7 @@
 # Exercism Command-line Interface (CLI)
 
 [![Build Status](https://travis-ci.org/exercism/cli.svg?branch=master)](https://travis-ci.org/exercism/cli)
+[![Go Report Card](https://goreportcard.com/badge/github.com/exercism/cli)](https://goreportcard.com/report/github.com/exercism/cli)
 
 The CLI is the link between the [Exercism][exercism] website and your local work environment. It lets you download exercises and submit your solution to the site.