add license badge to readme

Author: siherrmann

README.md

@@ -2,5 +2,6 @@
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/siherrmann/queuer.svg)](https://pkg.go.dev/github.com/siherrmann/queuer)
 [![Go Coverage](https://github.com/siherrmann/queuer/wiki/coverage.svg)](https://raw.githack.com/wiki/siherrmann/queuer/coverage.html)
+[![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/siherrmann/queuer/blob/master/LICENSE)
 
 Queueing package based on postgres written in Go.