Adding a downloads shield to the README ... 'cause why not

Update README.md

Author: avoidwork

README.md

@@ -1,6 +1,6 @@
 # filesize.js
 
-[![build status](https://secure.travis-ci.org/avoidwork/filesize.js.png)](http://travis-ci.org/avoidwork/filesize.js) [![Join the chat at https://gitter.im/avoidwork/filesize.js](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/avoidwork/filesize.js?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![build status](https://secure.travis-ci.org/avoidwork/filesize.js.png)](http://travis-ci.org/avoidwork/filesize.js) [![Join the chat at https://gitter.im/avoidwork/filesize.js](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/avoidwork/filesize.js?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![downloads](https://img.shields.io/npm/dt/filesize.svg)](https://www.npmjs.com/package/filesize)
 
 filesize.js provides a simple way to get a human readable file size string from a number (float or integer) or string.