add npm version badge

Author: Kelly Selden

README.md

@@ -1,5 +1,6 @@
 # FastBoot
 
+[![npm version](https://badge.fury.io/js/fastboot.svg)](https://badge.fury.io/js/fastboot)
 [![Build Status](https://travis-ci.org/ember-fastboot/fastboot.svg?branch=master)](https://travis-ci.org/ember-fastboot/ember-fastboot-server)
 
 FastBoot is a library for rendering Ember.js applications in Node.js.