docs: update README

Author: yourtion

README.md

@@ -1,5 +1,5 @@
 [![NPM version][npm-image]][npm-url]
-[![Test coverage][coveralls-image]][coveralls-url]
+[![codecov][codecov-image]][codecov-url]
 [![node version][node-image]][node-url]
 [![npm download][download-image]][download-url]
 [![npm license][license-image]][download-url]
@@ -8,8 +8,8 @@
 [npm-image]: https://img.shields.io/npm/v/erest.svg?style=flat-square
 [npm-url]: https://npmjs.org/package/erest
 [travis-image]: https://img.shields.io/travis/yourtion/node-erest.svg?style=flat-square
-[coveralls-image]: https://img.shields.io/coveralls/yourtion/node-erest.svg?style=flat-square
-[coveralls-url]: https://coveralls.io/r/yourtion/node-erest?branch=master
+[codecov-image]: https://codecov.io/github/yourtion/node-erest/graph/badge.svg?token=HRY0R63I95
+[codecov-url]: https://codecov.io/github/yourtion/node-erest
 [david-url]: https://david-dm.org/yourtion/node-erest
 [node-image]: https://img.shields.io/badge/node.js-%3E=_10-green.svg?style=flat-square
 [node-url]: http://nodejs.org/download/