docs(readme): update readme file (#5092)

* docs(readme): update readme file

Update the main readme file

* docs(readme): update readme file

README.md

@@ -22,8 +22,10 @@ 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 v6 of RxJS right now
-- [stable](https://github.com/ReactiveX/rxjs/commits/stable) - 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, 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`
+
+Most PRs should be made to **master**.
 
 ## Important