Updated README.md

README.md

@@ -188,6 +188,29 @@ Valitron\Validator::addRule('alwaysFail', function($field, $value, array $params
 }, 'Everything you do is wrong. You fail.');
 ```
 
+You can also use one-off rules that are only valid for the specified
+fields. 
+
+```php
+$v = new Valitron\Validator(array("foo" => "bar"));
+$v->rule(function($field, $value, $params, $fields) {
+    return true;
+}, "foo")->message("{field} failed...");
+```
+
+This is useful because such rules can have access to variables
+defined in the scope where the `Validator` lives. The Closure's
+signature is identical to `Validator::addRule` callback's
+signature. 
+
+If you wish to add your own rules that are not static (i.e.,
+your rule is not static and available to call `Validator` 
+instances), you need to use `Validator::addInstanceRule`. 
+This rule will take the same parameters as 
+`Validator::addRule` but it has to be called on a `Validator`
+instance.
+
+
 ## Alternate syntax for adding rules
 
 As the number of rules grows, you may prefer the alternate syntax