add notice

README.md

@@ -47,6 +47,7 @@ wget https://raw.github.com/zordius/lightncandy/master/src/lightncandy.php --no-
 LightnCandy requirement: PHP 5.3.0+ .
 
 **UPGRADE NOTICE**
+* The interface of custom helpers was changed from v0.13 . if you use this feature you may need to modify your custom helper functions.
 
 * Due to big change of render() debugging, the rendering support class `LCRun2` is renamed to `LCRun3`. If you compile templates as none standalone PHP code by LightnCandy v0.11 or before, you should compile these templates again. Or, you may run into `Class 'LCRun2' not found` error when you execute these old rendering functions.
 * Due to big change of variable name handling, the rendering support class `LCRun` is renamed to `LCRun2`. If you compile templates as none standalone PHP code by LightnCandy v0.9 or before, you should compile these templates again. Or, you may run into `Class 'LCRun' not found` error when you execute these old rendering functions.