Add coverage badge

README.md

@@ -5,6 +5,7 @@
 [![npm bundle size](https://img.shields.io/bundlephobia/minzip/axios-rate-limit?style=flat-square)](https://bundlephobia.com/package/axios-rate-limit@latest)
 [![build status](https://img.shields.io/github/actions/workflow/status/aishek/axios-rate-limit/node.js.yml
 )](https://github.com/aishek/axios-rate-limit/actions?query=branch%3Amaster++)
+[![code coverage](https://img.shields.io/coveralls/aishek/axios-rate-limit.svg?style=flat-square)](https://coveralls.io/r/aishek/axios-rate-limit)
 [![install size](https://img.shields.io/badge/dynamic/json?url=https://packagephobia.com/v2/api.json?p=axios-rate-limit&query=$.install.pretty&label=install%20size&style=flat-square)](https://packagephobia.now.sh/result?p=axios-rate-limit)
 [![known vulnerabilities](https://snyk.io/test/npm/axios-rate-limit/badge.svg)](https://snyk.io/test/npm/axios-rate-limit)