Add sts_endpoint to readme

README.md

@@ -15,6 +15,7 @@ If you think you’ve found a potential security issue, please do not post it in
 * `data_keys`: By default, the whole log record will be sent to Kinesis. If you specify key name(s) with this option, then only those keys and values will be sent to Kinesis. For example, if you are using the Fluentd Docker log driver, you can specify `data_keys log` and only the log message will be sent to Kinesis. If you specify multiple keys, they should be comma delimited.
 * `role_arn`: ARN of an IAM role to assume (for cross account access).
 * `endpoint`: Specify a custom endpoint for the Kinesis Streams API.
+* `sts_endpoint`: Specify a custom endpoint for the STS API; used to assume your custom role provided with `role_arn`.
 * `append_newline`: If you set append_newline as true, a newline will be addded after each log record.
 * `time_key`: Add the timestamp to the record under this key. By default the timestamp from Fluent Bit will not be added to records sent to Kinesis.
 * `time_key_format`: [strftime](http://man7.org/linux/man-pages/man3/strftime.3.html) compliant format string for the timestamp; for example, `%Y-%m-%dT%H:%M:%S%z`. This option is used with `time_key`.