doc(readme): mention JMX exporter (#12)
doc(readme): mention JMX exporter (#12)

...for other metrics from Kafka. [1] triggered this commit (and I think
Brian has a point.

[1] https://github.com/prometheus/docs/pull/673/files#r104324871
kawamuray/prometheus-kafka-consumer-group-exporter,kawamuray/prometheus-kafka-consumer-group-exporter