fix: update readme

Author: chantouchsek

README.md

@@ -123,6 +123,18 @@ It will create `$errors` object inside components.
 | **clear(field)**                     | clear specific error by field name.                              |
 | **onKeydown(event, 'baseFormName')** | event to clear error by event.target.name. (input the has name). |
 
+#### first(field || fields)
+
+In some situations, you may wish to run validation checks against a field only if that field is present in the data being validated. To quickly accomplish this, add the sometimes rule to your rule list:
+
+```js
+const errors = { name: [{ kh: ['This fist name field is required'] }] }
+
+$errors.first('name') // return array
+$errors.first('name[0]') // return object like
+$errors.first('name[0].kh') // return string like
+```
+
 ## Using with Vuex
 
 1.Create **proxies** folder or your prefer folder name for this