chore: adds Go reference link

Author: IAmRadek

README.md

@@ -2,6 +2,8 @@
 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)
+
 It supports nested structs, prefixes, defaults, required fields,
 slices, maps, arrays, pointers, durations, and custom (un)marshalers.
 A helper is provided to read variables from a .env file.