Add REUSE badge in README

Author: flashcode

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://github.com/flashcode/impostman/workflows/CI/badge.svg)](https://github.com/flashcode/impostman/actions?query=workflow%3A%22CI%22)
 [![MELPA](https://melpa.org/packages/impostman-badge.svg)](https://melpa.org/#/impostman)
 [![MELPA Stable](https://stable.melpa.org/packages/impostman-badge.svg)](https://stable.melpa.org/#/impostman)
+[![REUSE status](https://api.reuse.software/badge/github.com/flashcode/impostman)](https://api.reuse.software/info/github.com/flashcode/impostman)
 
 Postman collections and environments can be imported and used with these Emacs HTTP clients: