feat: adds Go Report Card

Author: IAmRadek

README.md

@@ -3,6 +3,7 @@ A small,
 dependency-free Go library for loading configuration from environment variables directly into your structs.
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/struct0x/envconfig.svg)](https://pkg.go.dev/github.com/struct0x/envconfig)
+[![Go Report Card](https://goreportcard.com/badge/github.com/struct0x/envconfig)](https://goreportcard.com/report/github.com/struct0x/envconfig)
 
 It supports nested structs, prefixes, defaults, required fields,
 slices, maps, arrays, pointers, durations, and custom (un)marshalers.