message: Update Airflow plugin README
  Airflow v1 exposed an experimental API [that got dropped](https://airflow.apache.org/docs/apache-airflow/stable/deprecated-rest-api-ref.html) later on. Let's add a note to the README making explicit users of Airflow v1 won't be able to integrate with this plugin unless they update.
diff: Welcome to the apache-airflow plugin!
  +This plugin serves as frontend to the REST API exposed by Apache Airflow.
  +Note only [Airflow v2 (and later)](https://airflow.apache.org/docs/apache-airflow/stable/deprecated-rest-api-ref.html) integrate with the plugin.
  +
  ## Feature Requests & Ideas
  - [ ] Add support for running multiple instances of Airflow for monitoring
  