add Codecov badge in README

Author: faissaloux

README.md

@@ -3,7 +3,7 @@
 This plugin afford math related expectations.
 
 
-[![Tests](https://github.com/faissaloux/pest-plugin-math/actions/workflows/tests.yml/badge.svg)](https://github.com/faissaloux/pest-plugin-math/actions/workflows/tests.yml) ![Packagist Version](https://img.shields.io/packagist/v/faissaloux/pest-plugin-math) ![Packagist License](https://img.shields.io/packagist/l/faissaloux/pest-plugin-math)
+[![Tests](https://github.com/faissaloux/pest-plugin-math/actions/workflows/tests.yml/badge.svg)](https://github.com/faissaloux/pest-plugin-math/actions/workflows/tests.yml) ![Codecov](https://img.shields.io/codecov/c/github/faissaloux/pest-plugin-math) ![Packagist Version](https://img.shields.io/packagist/v/faissaloux/pest-plugin-math) ![Packagist License](https://img.shields.io/packagist/l/faissaloux/pest-plugin-math)
 
 
 #### `toBeDivisibleBy()`