Add cloudflare/kafka_zookeeper_exporter to the list of exporters (prometheus#799)

Author: prymitive

content/docs/instrumenting/exporters.md

@@ -57,6 +57,7 @@ wide variety of JVM-based applications, for example [Kafka](http://kafka.apache.
 
 ### Messaging systems
    * [Kafka consumer group exporter](https://github.com/kawamuray/prometheus-kafka-consumer-group-exporter)
+   * [Kafka ZooKeeper exporter](https://github.com/cloudflare/kafka_zookeeper_exporter)
    * [NATS exporter](https://github.com/lovoo/nats_exporter)
    * [NSQ exporter](https://github.com/lovoo/nsq_exporter)
    * [RabbitMQ exporter](https://github.com/kbudde/rabbitmq_exporter)