Add sts_endpoint to readme

README.md

@@ -24,6 +24,7 @@ Run `make` to build `./bin/firehose.so`. Then use with Fluent Bit:
 * `data_keys`: By default, the whole log record will be sent to Kinesis. If you specify a 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 Firehose API.
+* `sts_endpoint`: Specify a custom endpoint for the STS API; used to assume your custom role provided with `role_arn`.
 * `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`.