Update README.md

Author: mcd8604

README.md

@@ -2,6 +2,6 @@
 
 ## Slash Commands
 - /mb, /messagebarker - Open/Close MessageBarker
-- /bm, /barkmsg, /barkmessage messageID
+- /bm, /barkmsg, /barkmessage messageID - Barks the message with a given ID if it exists
 - /messagebarker_minimap_hide - Hide the minimap button
 - /messagebarker_minimap_show - Show the minimap button