Update license link in readme.md

Author: mehw

readme.md

@@ -5,6 +5,7 @@
 [![MELPA](http://melpa.org/packages/resize-window-badge.svg)](http://melpa.org/#/resize-window)
 [![MELPA stable](http://stable.melpa.org/packages/resize-window-badge.svg)](http://stable.melpa.org/#/resize-window)
 [![Tag Version](https://img.shields.io/github/tag/dpsutton/resize-window.svg)](https://github.com/dpsutton/resize-window/tags)
+[![License](https://img.shields.io/:license-gpl3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0.en.html)
 
 ## What it is ##