message: docs: update branch information in README.md
diff: @@ -19,8 +19,8 @@ Reactive Extensions Library for JavaScript. This is a rewrite of [Reactive-Exten
  ## Versions In This Repository
  -- [master](https://github.com/ReactiveX/rxjs/commits/master) - This is all of the current, unreleased work, which is against v7 of RxJS right now
  -- [stable](https://github.com/ReactiveX/rxjs/tree/6.x) - This is the branch for the latest version you'd get if you do `npm install rxjs`
  +- [master](https://github.com/ReactiveX/rxjs/commits/master) - This is all of the current work, which is against v7 of RxJS right now
  +- [6.x](https://github.com/ReactiveX/rxjs/tree/6.x) - This is the branch for version 6.X
  Most PRs should be made to **master**.
  