message: docs(README): remove google docs URL
  The Google Doc's documentation has moved to MIGRATION.md
diff: Find out how up update to v6, **automatically update your TypeScript code**, and more!
  -- [Current home is a Google Doc](https://goo.gl/osWFzo)
  -- [Future home is MIGRATION.md](./MIGRATION.md)
  +- [Current home is MIGRATION.md](./MIGRATION.md)
  ### FOR V 5.X PLEASE GO TO [THE 5.0 BRANCH](https://github.com/ReactiveX/rxjs/tree/stable)
  