message: docs(README): Update link to migration document,
diff: Find out how up update to v6, **automatically update your TypeScript code**, and more!
  -- [Current home is MIGRATION.md](./MIGRATION.md)
  +- [Current home is MIGRATION.md](./docs_app/content/guide/v6/migration.md)
  ### FOR V 5.X PLEASE GO TO [THE 5.0 BRANCH](https://github.com/ReactiveX/rxjs/tree/stable)
  