in_dummy: Add flush_on_startup setting doc. (#1264)

Signed-off-by: Francisco Valente <[email protected]>

pipeline/inputs/dummy.md

@@ -17,6 +17,7 @@ The plugin supports the following configuration parameters:
 | Interval\_nsec | Set nanoseconds of time interval at which every message is generated. If set, `Rate` configuration will be ignored. Default: 0 |
 | Samples | If set, the events number will be limited. e.g. If Samples=3, the plugin only generates three events and stops. |
 | Copies | Number of messages to generate each time they are generated. Defaults to 1. |
+| Flush\_on\_startup | If set to `true`, the first dummy event is generated at startup. Default: `false` |
 
 ## Getting Started