chore: add CodeCov badge to README.md (#250)

Author: trivikr

README.md

@@ -1,5 +1,7 @@
 # AWS SDK for JavaScript V3 Developer Preview
-[![Build Status](https://travis-ci.org/aws/aws-sdk-js-v3.svg?branch=master)](https://travis-ci.org/aws/aws-sdk-js-v3) [![Greenkeeper badge](https://badges.greenkeeper.io/aws/aws-sdk-js-v3.svg)](https://greenkeeper.io/)
+[![Build Status](https://travis-ci.org/aws/aws-sdk-js-v3.svg?branch=master)](https://travis-ci.org/aws/aws-sdk-js-v3)
+[![codecov](https://codecov.io/gh/aws/aws-sdk-js-v3/branch/master/graph/badge.svg)](https://codecov.io/gh/aws/aws-sdk-js-v3)
+[![Greenkeeper badge](https://badges.greenkeeper.io/aws/aws-sdk-js-v3.svg)](https://greenkeeper.io/)
 
 The __AWS SDK for JavaScript V3 Developer Preview__ is a rewrite of V2 with some great new features. As with version 2, it enables you to easily work with [Amazon Web Services](https://aws.amazon.com/), but has been written in TypeScript and adds several frequently requested features, like modularized packages.