docs: add codecov badge to README

Author: mario-beltran-cs

README.md

@@ -15,6 +15,7 @@
 [![Package version][version-badge]][version-url]
 [![eslint-remote-tester][eslint-remote-tester-badge]][eslint-remote-tester-workflow]
 [![eslint-plugin-testing-library][package-health-badge]][package-health-url]
+[![codecov](https://codecov.io/gh/testing-library/eslint-plugin-testing-library/graph/badge.svg?token=IJd6ZogYPm)](https://codecov.io/gh/testing-library/eslint-plugin-testing-library)
 [![MIT License][license-badge]][license-url]
 <br />
 [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg?style=flat-square)](https://github.com/semantic-release/semantic-release)