Add Go report card badge to README (#15)

Author: barweiss

README.md

@@ -2,6 +2,7 @@
 
 [![Go](https://github.com/barweiss/go-tuple/actions/workflows/go.yml/badge.svg)](https://github.com/barweiss/go-tuple/actions/workflows/go.yml)
 [![Coverage Status](https://coveralls.io/repos/github/barweiss/go-tuple/badge.svg)](https://coveralls.io/github/barweiss/go-tuple)
+[![Go Report Card](https://goreportcard.com/badge/github.com/barweiss/go-tuple)](https://goreportcard.com/report/github.com/barweiss/go-tuple)
 [![Go Reference](https://pkg.go.dev/badge/github.com/barweiss/go-tuple.svg)](https://pkg.go.dev/github.com/barweiss/go-tuple)
 
 Go 1.18 tuple implementation.