docs(README): add browser test covergae matrix

relates to ReactiveX#998

Author: kwonoj

README.md

@@ -2,6 +2,8 @@
 [![Coverage Status](https://coveralls.io/repos/ReactiveX/RxJS/badge.svg?branch=master&service=github)](https://coveralls.io/github/ReactiveX/RxJS?branch=master)
 [![npm version](https://badge.fury.io/js/%40reactivex%2Frxjs.svg)](http://badge.fury.io/js/%40reactivex%2Frxjs)
 
+[![Selenium Test Status](https://saucelabs.com/browser-matrix/rxjs5.svg)](https://saucelabs.com/u/rxjs5)
+
 # RxJS 5 (beta)
 
 Reactive Extensions Library for JavaScript. This is a rewrite of [Reactive-Extensions/RxJS](https://github.com/Reactive-Extensions/RxJS) and is intended to supersede it once this is ready. This rewrite is meant to have better performance, better modularity, better debuggable call stacks, while staying mostly backwards compatible, with some breaking changes that reduce the API surface.