chore: add codecov badge to README

README.md

@@ -2,6 +2,7 @@
 
 [![CI](https://github.com/eclipse-zenoh/zenoh/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/eclipse-zenoh/zenoh/actions?query=workflow%3ACI+branch%3Amain++)
 [![Documentation Status](https://readthedocs.org/projects/zenoh-rust/badge/?version=latest)](https://zenoh-rust.readthedocs.io/en/latest/?badge=latest)
+[![codecov](https://codecov.io/github/eclipse-zenoh/zenoh/branch/main/graph/badge.svg?token=F8T4C8WPZD)](https://codecov.io/github/eclipse-zenoh/zenoh)
 [![Discussion](https://img.shields.io/badge/discussion-on%20github-blue)](https://github.com/eclipse-zenoh/roadmap/discussions)
 [![Discord](https://img.shields.io/badge/chat-on%20discord-blue)](https://discord.gg/2GJ958VuHs)
 [![License](https://img.shields.io/badge/License-EPL%202.0-blue)](https://choosealicense.com/licenses/epl-2.0/)