Document in array validation rule

Author: themsaid

validation.md

@@ -496,7 +496,8 @@ Below is a list of all available validation rules and their function:
 [Exists (Database)](#rule-exists)
 [Filled](#rule-filled)
 [Image (File)](#rule-image)
-[In](#rule-in)
+[In](#rule-in)
+[In Array](#rule-in-array)
 [Integer](#rule-integer)
 [IP Address](#rule-ip)
 [JSON](#rule-json)
@@ -660,7 +661,12 @@ The file under validation must be an image (jpeg, png, bmp, gif, or svg)
 <a name="rule-in"></a>
 #### in:_foo_,_bar_,...
 
-The field under validation must be included in the given list of values.
+The field under validation must be included in the given list of values.
+
+<a name="rule-in-array"></a>
+#### in_array:_anotherfield_
+
+The field under validation must exist in _anotherfield_'s values.
 
 <a name="rule-integer"></a>
 #### integer