doc: Update README.md

Author: jaywcjlove

README.md

@@ -4,6 +4,7 @@
 [![Build & Deploy](https://github.com/uiwjs/react-codemirror/workflows/Build%20&%20Deploy/badge.svg)](https://github.com/uiwjs/react-codemirror/actions)
 [![Open in unpkg](https://img.shields.io/badge/Open%20in-unpkg-blue)](https://uiwjs.github.io/npm-unpkg/#/pkg/@uiw/react-codemirror/file/README.md)
 [![npm version](https://img.shields.io/npm/v/@uiw/react-codemirror.svg)](https://www.npmjs.com/package/@uiw/react-codemirror)
+[![Coverage Status](https://coveralls.io/repos/github/uiwjs/react-codemirror/badge.svg?branch=master)](https://coveralls.io/github/uiwjs/react-codemirror?branch=master)
 
 CodeMirror component for React. Demo Preview: [@uiwjs.github.io/react-codemirror](https://uiwjs.github.io/react-codemirror/)