diff --git a/README.md b/README.md
index 9b9e888..4458940 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,7 @@ action "Send Default Message" {
 * VENUE - Optional. venue message
 * VIDEO - Optional. video message
 * DEBUG - Optional. enable debug mode
+* FORMAT - Optional. `markdown` or `html`
 
 ### Example