README: add a badge for Go Report Card
MODIFY
README.md
README.md
-# Gonum [![Build Status](https://travis-ci.org/gonum/gonum.svg?branch=master)](https://travis-ci.org/gonum/gonum) [![Coverage Status](https://coveralls.io/repos/gonum/gonum/badge.svg?branch=master&service=github)](https://coveralls.io/github/gonum/gonum?branch=master) [![GoDoc](https://godoc.org/gonum.org/v1/gonum?status.svg)](https://godoc.org/gonum.org/v1/gonum)
+# Gonum [![Build Status](https://travis-ci.org/gonum/gonum.svg?branch=master)](https://travis-ci.org/gonum/gonum) [![Coverage Status](https://coveralls.io/repos/gonum/gonum/badge.svg?branch=master&service=github)](https://coveralls.io/github/gonum/gonum?branch=master) [![GoDoc](https://godoc.org/gonum.org/v1/gonum?status.svg)](https://godoc.org/gonum.org/v1/gonum) [![Go Report Card](https://goreportcard.com/badge/github.com/gonum/gonum)](https://goreportcard.com/report/github.com/gonum/gonum)
## Issues

README: add a badge for Go Report Card