[README.md] add license badge

Author: chrissimpkins

README.md

@@ -3,6 +3,7 @@
 
 ## An OpenType table diff tool for fonts
 
+[![GitHub license](https://img.shields.io/github/license/source-foundry/fdiff?color=blue)](https://github.com/source-foundry/fdiff/blob/master/LICENSE)
 [![Build Status](https://travis-ci.org/source-foundry/fdiff.svg?branch=master)](https://travis-ci.org/source-foundry/fdiff)
 [![Build status](https://ci.appveyor.com/api/projects/status/eafvbkc4iyv78dip/branch/master?svg=true)](https://ci.appveyor.com/project/chrissimpkins/fdiff/branch/master)
 [![codecov](https://codecov.io/gh/source-foundry/fdiff/branch/master/graph/badge.svg)](https://codecov.io/gh/source-foundry/fdiff)