Update README.md

Author: vltansky

README.md

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 # Create HTML5 Boilerplate
 
 [![Coverage Status](https://coveralls.io/repos/github/h5bp/create-html5-boilerplate/badge.svg?branch=master)](https://coveralls.io/github/h5bp/create-html5-boilerplate?branch=master)
+[![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg?style=flat-square)](https://github.com/prettier/prettier)
 
 Quick start for HTML5 Boilerplate. Get up and running with one command.