docs: add codecov badge to README.md

README.md

@@ -3,6 +3,7 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/sincerefly/capybara)](https://goreportcard.com/report/github.com/sincerefly/capybara)
 <a href="https://opensource.org/licenses/MIT"><img src="https://img.shields.io/badge/license-MIT-_red.svg"></a>
 <a href="https://github.com/sincerefly/capybara/issues"><img src="https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat"></a>
+[![codecov](https://codecov.io/gh/sincerefly/capybara/graph/badge.svg?token=D8RGT9H0TU)](https://codecov.io/gh/sincerefly/capybara)
 
 A convenient tool for adding borders, watermarks to images, with possible future additions such as format conversion, image compression, and more.