Update README.md with npm total download badge

Author: chpapa

README.md

@@ -1,6 +1,7 @@
 # skygear
 
 [![npm](https://img.shields.io/npm/v/skygear.svg)](https://www.npmjs.com/package/skygear)
+[![npm](https://img.shields.io/npm/dt/skygear.svg)](https://www.npmjs.com/package/skygear)
 [![Build Status](https://travis-ci.org/SkygearIO/skygear-SDK-JS.svg?branch=master)](https://travis-ci.org/SkygearIO/skygear-SDK-JS)
 [![License](https://img.shields.io/npm/l/skygear.svg)](https://www.npmjs.com/package/skygear)