Add Kafka output readme (#1609)

plugins/outputs/kafka/README.md

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+# Kafka Producer Output Plugin
+
+This plugin writes to a [Kafka Broker](http://kafka.apache.org/07/quickstart.html) acting a Kafka Producer.
+
+```
+[[outputs.kafka]]
+  ## URLs of kafka brokers
+  brokers = ["localhost:9092"]
+  ## Kafka topic for producer messages
+  topic = "telegraf"
+  ## Telegraf tag to use as a routing key
+  ##  ie, if this tag exists, it's value will be used as the routing key
+  routing_tag = "host"
+
+  ## CompressionCodec represents the various compression codecs recognized by
+  ## Kafka in messages.
+  ##  0 : No compression
+  ##  1 : Gzip compression
+  ##  2 : Snappy compression
+  compression_codec = 0
+
+  ##  RequiredAcks is used in Produce Requests to tell the broker how many
+  ##  replica acknowledgements it must see before responding
+  ##   0 : the producer never waits for an acknowledgement from the broker.
+  ##       This option provides the lowest latency but the weakest durability
+  ##       guarantees (some data will be lost when a server fails).
+  ##   1 : the producer gets an acknowledgement after the leader replica has
+  ##       received the data. This option provides better durability as the
+  ##       client waits until the server acknowledges the request as successful
+  ##       (only messages that were written to the now-dead leader but not yet
+  ##       replicated will be lost).
+  ##   -1: the producer gets an acknowledgement after all in-sync replicas have
+  ##       received the data. This option provides the best durability, we
+  ##       guarantee that no messages will be lost as long as at least one in
+  ##       sync replica remains.
+  required_acks = -1
+
+  ##  The total number of times to retry sending a message
+  max_retry = 3
+
+  ## Optional SSL Config
+  # ssl_ca = "/etc/telegraf/ca.pem"
+  # ssl_cert = "/etc/telegraf/cert.pem"
+  # ssl_key = "/etc/telegraf/key.pem"
+  ## Use SSL but skip chain & host verification
+  # insecure_skip_verify = false
+
+  data_format = "influx"
+```
+
+### Required parameters:
+
+* `brokers`: List of strings, this is for speaking to a cluster of `kafka` brokers. On each flush interval, Telegraf will randomly choose one of the urls to write to. Each URL should just include host and port e.g. -> `["{host}:{port}","{host2}:{port2}"]`
+* `topic`: The `kafka` topic to publish to.
+
+
+### Optional parameters:
+
+* `routing_tag`:  if this tag exists, it's value will be used as the routing key
+* `compression_codec`: What level of compression to use: `0` -> no compression, `1` -> gzip compression, `2` -> snappy compression
+* `required_acks`: a setting for how may `acks` required from the `kafka` broker cluster.
+* `max_retry`: Max number of times to retry failed write
+* `ssl_ca`: SSL CA
+* `ssl_cert`: SSL CERT
+* `ssl_key`: SSL key
+* `insecure_skip_verify`: Use SSL but skip chain & host verification (default: false)
+* `data_format`: [About Telegraf data formats](https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_OUTPUT.md)