docs: add codecov badge (#1600)

Author: vil02

Diff for: README.md

@@ -10,6 +10,7 @@ JavaScript Repository of TheAlgorithms, which implements various algorithms and
 
 [![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/TheAlgorithms/JavaScript)
 [![Checks][checks]][actions]
+[![codecov](https://codecov.io/gh/TheAlgorithms/JavaScript/graph/badge.svg?token=8VeZwL31KZ)](https://codecov.io/gh/TheAlgorithms/JavaScript)
 [![Contributions Welcome][welcome]](CONTRIBUTING.md)
 [![standard.js][standard-logo]][standard-js]
 [![Discord chat][chat]][discord-server]