Document how to configure Kafka topics

[cer]

See https://eventuate-users.slack.com/archives/C3L9MNMQC/p1589999456102900

I assume that you are using Kafka. You can specify the default at the broker level: http://kafka.apache.org/documentation/#num.partitions or you can create/reconfigure a topic with the desired number using kafka-topics.sh. For example, see this answer on stack overflow: https://stackoverflow.com/questions/22152269/how-to-specify-number-of-partitions-on-kafka-2-8