Proposal: Add shields io npm version badge (#3099)

readme.md

 <img width="112" alt="screen shot 2016-10-25 at 2 37 27 pm" src="https://cloud.githubusercontent.com/assets/13041/19686250/971bf7f8-9ac0-11e6-975c-188defd82df1.png">
 
+[![NPM version](https://img.shields.io/npm/v/next.svg)](https://www.npmjs.com/package/next) 
 [![Build Status](https://travis-ci.org/zeit/next.js.svg?branch=master)](https://travis-ci.org/zeit/next.js)
 [![Build status](https://ci.appveyor.com/api/projects/status/gqp5hs71l3ebtx1r/branch/master?svg=true)](https://ci.appveyor.com/project/arunoda/next-js/branch/master)
 [![Coverage Status](https://coveralls.io/repos/zeit/next.js/badge.svg?branch=master)](https://coveralls.io/r/zeit/next.js?branch=master)