Updated README.md for GPLv3 license info.

README.md

-[![License: CC BY-NC-SA 4.0](https://img.shields.io/badge/License-CC%20BY--NC--SA%204.0-lightgrey.svg)](http://creativecommons.org/licenses/by-nc-sa/4.0/)
+[![License: GPLv3](https://img.shields.io/badge/license-GPLv3-blue)](https://www.gnu.org/licenses/gpl-3.0.html)
 
 # ![prescott logo](./images/logo-1.png)
 # A QUICK INTRODUCTION TO PRESCOTT