npm downloads (#3040)

Add ````npm dm```.

README.md

@@ -1,5 +1,6 @@
 [![Travis CI Build Status](https://travis-ci.org/gatsbyjs/gatsby.svg?branch=master)](https://travis-ci.org/gatsbyjs/gatsby)
 [![npm package](https://img.shields.io/npm/v/gatsby.svg?style=flat-square)](https://www.npmjs.org/package/gatsby)
+[![npm package](https://img.shields.io/npm/dm/gatsby.svg)](https://www.npmjs.org/package/gatsby)
 [![Join the community on Spectrum](https://withspectrum.github.io/badge/badge.svg)](https://spectrum.chat/gatsby-js)
 [![gatsby channel on discord](https://img.shields.io/badge/discord-gatsby%40reactiflux-738bd7.svg?style=flat-square)](https://discord.gg/0ZcbPKXt5bVoxkfV)
 [![Twitter Follow](https://img.shields.io/twitter/follow/gatsbyjs.svg?style=social)](https://twitter.com/gatsbyjs)