Add README.md for mqtt output (#3764)

plugins/outputs/mqtt/README.md

@@ -0,0 +1,54 @@
+# MQTT Producer Output Plugin
+
+This plugin writes to a [MQTT Broker](http://http://mqtt.org/) acting as a mqtt Producer.
+
+```
+[[outputs.mqtt]]
+  ## URLs of mqtt brokers
+  servers = ["localhost:1883"]
+  
+  ## topic for producer messages
+  topic = "telegraf"
+  
+  ## QoS policy for messages
+  qos = 2
+  
+  ## username and password to connect MQTT server.
+  # username = "telegraf"
+  # password = "metricsmetricsmetricsmetrics"
+   
+  ## client ID, if not set a random ID is generated
+  # client_id = ""
+    
+  ## Timeout for write operations. default: 5s
+  # timeout = "5s"
+  ## 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 to output.
+  data_format = "influx"
+
+
+```
+
+### Required parameters:
+
+* `servers`: List of strings, this is for speaking to a cluster of `mqtt` 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_prefix`: The `mqtt` topic prefix to publish to. MQTT outputs send metrics to this topic format "<topic_prefix>/<hostname>/<pluginname>/" ( ex: prefix/web01.example.com/mem)
+* `qos`: The `mqtt` QoS policy for sending messages. See https://www.ibm.com/support/knowledgecenter/en/SSFKSJ_9.0.0/com.ibm.mq.dev.doc/q029090_.htm for details.
+
+### Optional parameters:
+* `username`: The username to connect MQTT server.
+* `password`: The password to connect MQTT server.
+* `client_id`: The unique client id to connect MQTT server. If this paramater is not set then a random ID is generated.
+* `timeout`: Timeout for write operations. default: 5s
+* `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)