Update README

README.md

@@ -167,6 +167,7 @@ Each type also has additional properties.
    The string type validates the item as a string. it also has the following properties:   
    * **constraints** [*array*] : Exta constraints are:
       * **length** [*array*] : the minimum and maximum length of the string as a numeric array in the format [min, max]. If you don't want to set a value set it to null. So [1,null] will be a string with a minimum of one character but no maximum set.
+      * **enum** [*array*] : the possible values of the string. so [FOO, BAR] means that the string can only be one of those values and nothing else
 
 - **number**