Add a description for unrecoverable_error_code

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

README.md

@@ -558,6 +558,8 @@ You need to install rdkafka gem.
   # load of both Fluentd and Kafka when excessive messages are attempted
   # to send. Default is no limit.
   max_enqueue_bytes_per_second (integer) :default => nil
+  unrecoverable_error_codes (array) :default => ["topic_authorization_failed", "msg_size_too_large"]
+
  </match>
 
 `rdkafka2` supports `discard_kafka_delivery_failed_regex` parameter: