Add npm version badge to readme

Author: mrhwick

README.md

@@ -1,5 +1,6 @@
 # [bookshelf.js](http://bookshelfjs.org)
 
+[![NPM Version](https://img.shields.io/npm/v/bookshelf.svg?style=flat)](https://www.npmjs.com/package/bookshelf)
 [![Build Status](https://travis-ci.org/bookshelf/bookshelf.svg?branch=master)](https://travis-ci.org/bookshelf/bookshelf)
 [![Dependency Status](https://david-dm.org/bookshelf/bookshelf/status.svg)](https://david-dm.org/bookshelf/bookshelf)
 [![devDependency Status](https://david-dm.org/bookshelf/bookshelf/dev-status.svg)](https://david-dm.org/bookshelf/bookshelf?type=dev)