add go report card

README.md

@@ -1,4 +1,4 @@
-# errors [![Travis-CI](https://travis-ci.org/pkg/errors.svg)](https://travis-ci.org/pkg/errors) [![GoDoc](https://godoc.org/github.com/pkg/errors?status.png)](http://godoc.org/github.com/pkg/errors)
+# errors [![Travis-CI](https://travis-ci.org/pkg/errors.svg)](https://travis-ci.org/pkg/errors) [![GoDoc](https://godoc.org/github.com/pkg/errors?status.png)](http://godoc.org/github.com/pkg/errors) [![Report card](https://goreportcard.com/badge/github.com/pkg/errors)](https://goreportcard.com/report/github.com/pkg/errors)
 
 Package errors implements functions for manipulating errors.