docs(readme): add codecov badge

Author: jsjoeio

README.md

@@ -1,5 +1,6 @@
 # code-server · [!["GitHub Discussions"](https://img.shields.io/badge/%20GitHub-%20Discussions-gray.svg?longCache=true&logo=github&colorB=purple)](https://github.com/cdr/code-server/discussions) [!["Join us on Slack"](https://img.shields.io/badge/join-us%20on%20slack-gray.svg?longCache=true&logo=slack&colorB=brightgreen)](https://cdr.co/join-community) [![Twitter Follow](https://img.shields.io/twitter/follow/CoderHQ?label=%40CoderHQ&style=social)](https://twitter.com/coderhq)
 
+[![codecov](https://codecov.io/gh/cdr/code-server/branch/main/graph/badge.svg?token=5iM9farjnC)](https://codecov.io/gh/cdr/code-server)
 [![See latest docs](https://img.shields.io/static/v1?label=Docs&message=see%20latest%20&color=blue)](https://github.com/cdr/code-server/tree/v3.9.3/docs)
 
 Run [VS Code](https://github.com/Microsoft/vscode) on any machine anywhere and access it in the browser.