document subscription via risingwave-py

[neverchanje]

The document of subscription lacks instructions on using a risingwave-py for a simpler exactly-once implementation.

threading.Thread(
    target=lambda: mv.on_change(
        handler = simple_event_handler,
        output_format=OutputFormat.DATAFRAME,
        # If set to True, progress of the subscription will be saved and can be recovered on python application crashed
        persist_progress=True, 
        max_batch_size = 10)
    ).start()

With persist_progress=True,, we can support exactly-once delivery, without worrying data loss after failure recovery.