message: docs(readme): Update readme to mention beta instead of alpha
diff: [![Selenium Test Status](https://saucelabs.com/browser-matrix/rxjs5.svg)](https://saucelabs.com/u/rxjs5)
  -# RxJS 6 Alpha
  +# RxJS 6 Beta
  ## FOR V 5.X PLEASE GO TO [STABLE BRANCH](https://github.com/ReactiveX/rxjs/tree/stable)
  @@ -33,7 +33,7 @@ By contributing or commenting on issues in this repository, whether you've read
  ### ES6 via npm
  ```sh
  -npm install rxjs@alpha
  +npm install rxjs@beta
  ```
  To import only what you need by patching (this is useful for size-sensitive bundling):
  @@ -52,7 +52,7 @@ range(1, 200)
  To install this library for CommonJS (CJS) usage, use the following command:
  ```sh
  -npm install rxjs@alpha
  +npm install rxjs@beta
  ```
  (Note: destructuring available in Node 8+)
  