adding optional custom cssClass

Author: IbrahimHd

README.md

@@ -68,6 +68,7 @@ For both date and time input
 | datetime | moment.js datetime object or a datetime string | none | no |
 | format | moment.js compatible date/time format | none | no |
 | placeholder | Placeholder is shown when input object is undefined | none | no |
+| cssClass | custom css class name | pre-defined style | no |
 | on-change | Handler function that is fired on change of datetime object | none | no |
 
 ## Dependencies