chore: add npm package badge to README (#285)

Author: z11h

README.md

@@ -1,5 +1,5 @@
 # Material Design for Angular 2
-
+[![NPM Version](https://badge.fury.io/js/%40angular2-material%2Fcore.svg)](https://www.npmjs.com/package/%2540angular2-material%2Fcore)
 [![Build Status](https://travis-ci.org/angular/material2.svg?branch=master)](https://travis-ci.org/angular/material2)
 [![Gitter](https://badges.gitter.im/angular/material2.svg)](https://gitter.im/angular/material2?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)