Add badge telling current version on NPM

Author: TheMaxoor

README.md

@@ -1,6 +1,7 @@
 # Webpack Encore: A Simple & Powerful Webpack API
 
 [![Build Status](https://travis-ci.org/symfony/webpack-encore.svg?branch=master)](https://travis-ci.org/symfony/webpack-encore)
+[![NPM Version](https://badge.fury.io/js/%40symfony%2Fwebpack-encore.svg)](https://badge.fury.io/js/%40symfony%2Fwebpack-encore)
 
 Webpack Encore is a simpler way to integrate [Webpack](https://webpack.js.org/) into your
 application. It *wraps* Webpack, giving you a clean & powerful API