docs(readme): add code coverage badge (#577)

README.md

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 # AWS Lambda Powertools (Python)
 
 ![Build](https://github.com/awslabs/aws-lambda-powertools/workflows/Powertools%20Python/badge.svg?branch=master)
+[![codecov.io](https://codecov.io/github/awslabs/aws-lambda-powertools-python/branch/develop/graphs/badge.svg)](https://app.codecov.io/gh/awslabs/aws-lambda-powertools-python)
 ![PythonSupport](https://img.shields.io/static/v1?label=python&message=3.6%20|%203.7|%203.8&color=blue?style=flat-square&logo=python) ![PyPI version](https://badge.fury.io/py/aws-lambda-powertools.svg) ![PyPi monthly downloads](https://img.shields.io/pypi/dm/aws-lambda-powertools)
 
 A suite of Python utilities for AWS Lambda functions to ease adopting best practices such as tracing, structured logging, custom metrics, and more. ([AWS Lambda Powertools Java](https://github.com/awslabs/aws-lambda-powertools-java) is also available).