Added Code Coverage badge to Readme

Author: ahnv

README.md

@@ -3,6 +3,7 @@
 
 [![Python CI](<https://github.com/imagekit-developer/imagekit-python/workflows/Python%20CI/badge.svg>)](https://github.com/imagekit-developer/imagekit-python/)
 [![imagekitio](<https://img.shields.io/pypi/v/imagekitio.svg>)](https://pypi.org/project/imagekitio)
+[![codecov](https://codecov.io/gh/imagekit-developer/imagekit-python/branch/master/graph/badge.svg?token=CwKWqBIlCu)](https://codecov.io/gh/imagekit-developer/imagekit-python)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 [![Twitter Follow](https://img.shields.io/twitter/follow/imagekitio?label=Follow&style=social)](https://twitter.com/ImagekitIo)