Added codecov badge

Author: delvedor

README.md

@@ -2,7 +2,7 @@
 
 # @elastic/elasticsearch
 
-[![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat)](http://standardjs.com/)  [![Build Status](https://clients-ci.elastic.co/job/elastic+elasticsearch-js+master/badge/icon)](https://clients-ci.elastic.co/job/elastic+elasticsearch-js+master/)  [![NPM downloads](https://img.shields.io/npm/dm/@elastic/elasticsearch.svg?style=flat)](https://www.npmjs.com/package/@elastic/elasticsearch)
+[![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat)](http://standardjs.com/)  [![Build Status](https://clients-ci.elastic.co/job/elastic+elasticsearch-js+master/badge/icon)](https://clients-ci.elastic.co/job/elastic+elasticsearch-js+master/)  [![codecov](https://codecov.io/gh/elastic/elasticsearch-js/branch/master/graph/badge.svg)](https://codecov.io/gh/elastic/elasticsearch-js)  [![NPM downloads](https://img.shields.io/npm/dm/@elastic/elasticsearch.svg?style=flat)](https://www.npmjs.com/package/@elastic/elasticsearch)
 
 The official Node.js client for Elasticsearch.