Add NPM version icon

Author: angelozerr

README.md

@@ -1,6 +1,7 @@
 # Acorn-JSX
 
 [![Build Status](https://travis-ci.org/RReverser/acorn-jsx.svg?branch=master)](https://travis-ci.org/RReverser/acorn-jsx)
+[![NPM version](https://img.shields.io/npm/v/acorn-jsx.svg)](https://www.npmjs.org/package/acorn-jsx)
 
 This is plugin for [Acorn](http://marijnhaverbeke.nl/acorn/) - a tiny, fast JavaScript parser, written completely in JavaScript.