Add Example for Apache Kafka (rdkafka)

Thank you very much for this amazing library :)

I tried a lot of places but can't find a single working example of actix-web and Kafka ( i mostly interested in https://docs.rs/rdkafka/0.24.0/rdkafka/producer/future_producer/struct.FutureProducer.html so heavy tasks can be delegated to individual consumers like Sending an email or generating thumbnail etc..)