add license badge to README

Author: josesho

README.md

@@ -1,6 +1,7 @@
 # DABEST-Python
 [![Travis CI](https://travis-ci.org/ACCLAB/DABEST-python.svg?branch=master)](https://travis-ci.org/ACCLAB/DABEST-python)
 [![PyPI version](https://badge.fury.io/py/dabest.svg)](https://badge.fury.io/py/dabest)
+[![License](https://img.shields.io/badge/License-BSD%203--Clause--Clear-blue.svg)](https://spdx.org/licenses/BSD-3-Clause-Clear.html)
 
 ## About