add CI badge

Author: fancaiyu

README.md

@@ -1,3 +1,4 @@
 # MCintegration
 [![alpha](https://img.shields.io/badge/docs-alpha-blue.svg)](https://numericaleft.github.io/MCintegration.py/)
+[![Build Status](https://github.com/numericalEFT/MCIntegration.py/workflows/CI/badge.svg)](https://github.com/numericalEFT/MCIntegration.py/actions)
 [![codecov](https://codecov.io/gh/numericalEFT/MCintegration.py/graph/badge.svg?token=851N2CNOTN)](https://codecov.io/gh/numericalEFT/MCintegration.py)