Updated readme for json support

plugins/outputs/kinesis/README.md

@@ -59,6 +59,24 @@ Kinesis. Right now there are three supported formats; string, json and custom.
 
 String is defined using the default Point.String() value and translated to []byte for the Kinesis stream.
 
+#### json
+
+Json format serialized Telegraf metrics in json format. The format is:
+
+    {
+       "fields":{
+          "field_1":30,
+          "field_2":4,
+          "field_N":59,
+          "n_images":660
+       },
+       "name":"docker",
+       "tags":{
+          "host":"raynor"
+       },
+       "timestamp":1458229140
+    }
+
 #### custom
 
 Custom is a string defined by a number of values in the FormatMetric() function.