Update readme.md

Former-commit-id: ca141be

Author: joppuyo

readme.md

@@ -2,6 +2,7 @@
 
 [![GitHub Workflow Status](https://img.shields.io/github/workflow/status/joppuyo/jpeg-xl-encode/Test?label=tests&logo=github)](https://github.com/joppuyo/jpeg-xl-encode/actions)
 [![Packagist Version](https://img.shields.io/packagist/v/joppuyo/jpeg-xl-encode)](https://packagist.org/packages/joppuyo/jpeg-xl-encode)
+[![codecov](https://codecov.io/gh/joppuyo/jpeg-xl-encode/branch/main/graph/badge.svg?token=KBTKSRNEG6)](https://codecov.io/gh/joppuyo/jpeg-xl-encode)
 [![Packagist PHP Version Support](https://img.shields.io/packagist/php-v/joppuyo/jpeg-xl-encode)](https://packagist.org/packages/joppuyo/jpeg-xl-encode)
 [![Packagist License](https://img.shields.io/packagist/l/joppuyo/jpeg-xl-encode)](https://packagist.org/packages/joppuyo/jpeg-xl-encode)