Add ReportCard badge.

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/jlaffaye/ftp.svg?branch=master)](https://travis-ci.org/jlaffaye/ftp)
 [![Coverage Status](https://coveralls.io/repos/jlaffaye/ftp/badge.svg?branch=master&service=github)](https://coveralls.io/github/jlaffaye/ftp?branch=master)
+[![Go ReportCard](http://goreportcard.com/badge/jlaffaye/ftp)](http://goreportcard.com/report/jlaffaye/ftp)
 
 A FTP client package for Go