add ci badge (#114)

* add ci badge

* typo

Author: juanitorduz

README.md

@@ -1,5 +1,6 @@
 # CausalPy
 
+![Build](https://github.com/pymc-labs/CausalPy/workflows/ci/badge.svg)
 [![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)
 ![GitHub Repo stars](https://img.shields.io/github/stars/pymc-labs/causalpy?style=social)