add Go Report Card badge

Author: kdomanski

README.md

@@ -1,6 +1,7 @@
 ## iso9660
 [![Go Reference](https://pkg.go.dev/badge/github.com/kdomanski/iso9660.svg)](https://pkg.go.dev/github.com/kdomanski/iso9660)
 [![codecov](https://codecov.io/gh/kdomanski/iso9660/branch/master/graph/badge.svg?token=14MJSZYZ24)](https://codecov.io/gh/kdomanski/iso9660)
+[![Go Report Card](https://goreportcard.com/badge/github.com/kdomanski/iso9660)](https://goreportcard.com/report/github.com/kdomanski/iso9660)
 
 A package for reading and creating ISO9660