title: Any example of Airflow FileSensor?
body: <p>Whoever can please point me to an example of how to use Airflow FileSensor?
  I've googled and haven't found anything yet. Any example would be sufficient. My use case is quite simple: </p>
  
  <p>Wait for a scheduled DAG to drop a file in a path, FileSensor task picks it up, read content and process it.</p>