[README.md] add license badge

Author: chrissimpkins

README.md

@@ -1,6 +1,7 @@
 ## font-size
 
 [![PyPI](https://img.shields.io/pypi/v/font-size?color=blueviolet&label=PyPI&logo=python&logoColor=white)](https://pypi.org/project/font-size/)
+[![GitHub license](https://img.shields.io/github/license/source-foundry/font-size?color=blue)](https://github.com/source-foundry/font-size/blob/master/LICENSE)
 [![Python CI](https://github.com/source-foundry/font-size/actions/workflows/py-ci.yml/badge.svg)](https://github.com/source-foundry/font-size/actions/workflows/py-ci.yml)
 [![Python Lints](https://github.com/source-foundry/font-size/actions/workflows/py-lint.yml/badge.svg)](https://github.com/source-foundry/font-size/actions/workflows/py-lint.yml)