[docs] Add saucelabs browser matrix in README (#1035)

Author: darrachequesne

README.md

@@ -8,6 +8,8 @@
 ![Downloads](http://img.shields.io/npm/dm/socket.io-client.svg?style=flat)
 [![](http://slack.socket.io/badge.svg?)](http://slack.socket.io)
 
+[![Sauce Test Status](https://saucelabs.com/browser-matrix/socket.svg)](https://saucelabs.com/u/socket)
+
 ## How to use
 
 A standalone build of `socket.io-client` is exposed automatically by the