update readme

Author: annikoff

README.md

@@ -15,6 +15,8 @@ To put a field ID in the formula, double-click on an item in the list of availab
 > - Be careful with code in formula, if it would wrong your application can be crashed
 > - This plugin has been tested on Redmine v2.6.7 with RoR v3.2.22 and Redmine v3.1 with RoR v4.2.3
 > - There are six types of output format: "int", "float", "string", "link", "datetime", "bool", "percentage"
+> - If a computed custom field was created after creating of customized object you need to re-save an object to evaluate computations
+> - After updating of formula customized objects should be re-saved
 
 ### Installation: