feat: add prettier badge

Author: dp3000

README.md

@@ -1,5 +1,6 @@
 [![Project Status](http://opensource.box.com/badges/active.svg)](http://opensource.box.com/badges/)
 [![Build Status](https://travis-ci.com/box/box-salesforce-sdk.svg?branch=master)](https://travis-ci.com/box/box-salesforce-sdk)
+[![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 [![Commitizen friendly](https://img.shields.io/badge/commitizen-friendly-brightgreen.svg)](http://commitizen.github.io/cz-cli/)
 
 # Box Salesforce SDK