README update

Author: ecwws

README.md

@@ -13,23 +13,31 @@ The purpose of this filter is to make sure the @timestamp field exists in the
 record which is necessary for the record to be indexed properly by
 elasticsearch.
 
-* If *@timestamp* field already exists, it will ensure the format is correct
+* If `@timestamp` field already exists, it will ensure the format is correct
   by parse and convert to format '%Y-%m-%dT%H:%M:%S%z'. **As of version 0.2.4, it
   will support epoch second / epoch millis format as a valid timestamp value. If
   such value is detected, it will be converted to iso8601 format for easier
   consumption of elasticsearch when dynamic mapping is used.**
 
-* If a field named *timestamp* or *time* or *syslog_timestamp* exists, it will
-  parse that field and conver it to format '%Y-%m-%dT%H:%M:%S%z' then store it
-  in *@timestamp* field.
+* If a field named `timestamp` or `time` or `syslog_timestamp` exists, it will
+  parse that field and conver it to format '%Y-%m-%dT%H:%M:%S.%L%z' then store it
+  in `@timestamp` field. In addition, a field `fluent_converted_timestamp`
+  is added to the object with the same value.
 
 * If none of the above field exists, it will insert current time in
-  '%Y-%m-%dT%H:%M:%S%z' format as the *@timestamp* field.
+  '%Y-%m-%dT%H:%M:%S.%L%z' format as the `@timestamp` field. A field
+  `fluent_added_timestamp` is added to the object with same value.
+
+## (>=0.2.6) Subsecond Precision
+
+`subsecond_precision` controls the subsecond precision during the conversion.
+Default value is set to `3` (millisecond).
 
 ## Usage
 
 ```
 <filter **>
   type elasticsearch_timestamp_check
+  subsecond_precision 3
 </filter>
 ```