Update `README.md` protobuf option section.

[riz0id]

The documentation on grpc-mqtt's custom protobuf options in the README.md:

• [ ] The boolean option haskell.grpc.mqtt.batched_stream.
• [ ] The boolean option haskell.grpc.mqtt.batched_stream_service.
• [ ] The boolean option haskell.grpc.mqtt.batched_stream_file.
• [ ] The CLevel option haskell.grpc.mqtt.client_clevel.
• [ ] The CLevel option haskell.grpc.mqtt.client_clevel_service.
• [ ] The CLevel option haskell.grpc.mqtt.client_clevel_file.
• [ ] The CLevel option haskell.grpc.mqtt.server_clevel.
• [ ] The CLevel option haskell.grpc.mqtt.server_clevel_service.
• [ ] The CLevel option haskell.grpc.mqtt.server_clevel_file.

I plan on opening a PR for this, this issue is just to keep a reminder for myself.