#55 add PyPI version badge

Author: drbenvincent

README.md

@@ -1,6 +1,7 @@
 # CausalPy
 
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
+[![PyPI version](https://badge.fury.io/py/CausalPy.svg)](https://badge.fury.io/py/CausalPy)
 
 A Python package focussing on causal inference in quasi-experimental settings. The package allows for sophisticated Bayesian model fitting methods to be used in addition to traditional OLS.