Update README: doas option

Author: civitaspo

README.md

@@ -19,6 +19,7 @@ A File Output Plugin for Embulk to write HDFS.
 - **rewind_seconds** When you use Date format in path_prefix property(like `/tmp/embulk/%Y-%m-%d/out`), the format is interpreted by using the time which is Now minus this property. (int, default: `0`)
 - **overwrite** overwrite files when the same filenames already exists (boolean, default: `false`)
     - *caution*: even if this property is `true`, this does not mean ensuring the idempotence. if you want to ensure the idempotence, you need the procedures to remove output files after or before running. 
+- **doas** username which access to Hdfs (string, default: executed user)
 
 ## Example