readme: added goreportcard

Author: mxschmitt

README.md

@@ -3,6 +3,7 @@
 [![GoDoc](https://godoc.org/github.com/mxschmitt/golang-combinations?status.svg)](https://godoc.org/github.com/mxschmitt/golang-combinations)
 [![Build Status](https://travis-ci.com/mxschmitt/golang-combinations.svg?branch=master)](https://travis-ci.com/mxschmitt/golang-combinations)
 [![Coverage Status](https://coveralls.io/repos/github/mxschmitt/golang-combinations/badge.svg?branch=master)](https://coveralls.io/github/mxschmitt/golang-combinations?branch=master)
+[![Go Report Card](https://goreportcard.com/badge/github.com/mxschmitt/golang-combinations)](https://goreportcard.com/report/github.com/mxschmitt/golang-combinations)
 [![License](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 
 This package provides a method to generate all ordered combinations out of a given string array.