add note for Extant rule

docs/validate.md

@@ -98,6 +98,14 @@ Validates the value as loosely equal (`==') to a specified value.
 $filter->validate('field')->is('equalToValue', $other_value);
 ```
 
+## extant
+
+Validates thhat the field exists in the subject, even if it is null or blank.
+
+```php
+$filter->validate('field')->is('extant');
+```
+
 ## float
 
 Validates the value as representing a float.