Add multiple_of validation rule

validation.md

@@ -639,6 +639,7 @@ Below is a list of all available validation rules and their function:
 [MIME Types](#rule-mimetypes)
 [MIME Type By File Extension](#rule-mimes)
 [Min](#rule-min)
+[Multiple Of](#multiple-of)
 [Not In](#rule-not-in)
 [Not Regex](#rule-not-regex)
 [Nullable](#rule-nullable)
@@ -983,6 +984,11 @@ A full listing of MIME types and their corresponding extensions may be found at
 
 The field under validation must have a minimum _value_. Strings, numerics, arrays, and files are evaluated in the same fashion as the [`size`](#rule-size) rule.
 
+<a name="multiple-of"></a>
+#### multiple_of:_value_
+
+The field under validation must be a multiple of _value_. This can be useful when validating a number input that utilises the `step` attribute.
+
 <a name="rule-not-in"></a>
 #### not_in:_foo_,_bar_,...