docs: add code style prettier badge (#254)

Author: trivikr

README.md

@@ -2,6 +2,7 @@
 [![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/)
+[![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 
 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.