Add code coverage on readme

Author: Mubelotix

README.md

@@ -24,6 +24,7 @@
   <a href="https://github.com/meilisearch/meilisearch-rust/blob/main/LICENSE"><img src="https://img.shields.io/badge/license-MIT-informational" alt="License"></a>
   <a href="https://github.com/meilisearch/meilisearch/discussions" alt="Discussions"><img src="https://img.shields.io/badge/github-discussions-red" /></a>
   <a href="https://ms-bors.herokuapp.com/repositories/62"><img src="https://bors.tech/images/badge_small.svg" alt="Bors enabled"></a>
+  <a href="https://codecov.io/gh/meilisearch/meilisearch-rust" ><img src="https://codecov.io/gh/meilisearch/meilisearch-rust/graph/badge.svg?token=NVO9OI8JMG"/></a>
 </p>
 
 <p align="center">⚡ The Meilisearch API client written for Rust 🦀</p>