Añadida badge de la licencia al README.md

Author: ibaimoya

README.md

@@ -3,6 +3,7 @@ poolobject
 
 [![codecov](https://codecov.io/gh/Nicop17/poolobject/graph/badge.svg?token=D6XRI5RQA1)](https://codecov.io/gh/Nicop17/poolobject)
 [![Java CI with Maven](https://github.com/Nicop17/poolobject/actions/workflows/Java17CImaven.yml/badge.svg)](https://github.com/Nicop17/poolobject/actions/workflows/Java17CImaven.yml)
+[![License: GPL v2](https://img.shields.io/badge/License-GPL%20v2-blue.svg)](https://opensource.org/licenses/GPL-2.0)
 [![Wiki Home](https://img.shields.io/badge/Wiki-Home-blue?style=flat-square)](https://github.com/Nicop17/poolobject/wiki)