document the undocumented 'loadingPlaceholder' property

README.md

@@ -397,6 +397,7 @@ function onInputKeyDown(event) {
 	scrollMenuIntoView |	bool	|	true		|	whether the viewport will shift to display the entire menu when engaged
 	searchable 	|	bool	|	true		|	whether to enable searching feature or not
 	searchPromptText |	string\|node	|	'Type to search'	|	label to prompt for search input
+	loadingPlaceholder |	string\|node	|	'Loading...'	|	label to prompt for loading search result
 	tabSelectsValue	|	bool	|	true	|	whether to select the currently focused value when the `[tab]` key is pressed
 	value 		|	any	|	undefined	|	initial field value
 	valueKey	|	string	|	'value'		|	the option property to use for the value