README.md in strong_json-0.0.3 vs README.md in strong_json-0.0.4

@@ -65,9 +65,18 @@
 * `boolean` The value must be `true` or `false`
 * `numeric` The value must be an instance of `Numeric` or a string which represents a number
 * `any` Any value except `nil` is accepted
 * `prohibited` Any value will be rejected
 
+### Shortcuts
+
+There are some alias for `optional(base)`, where base is base types, as the following:
+
+* `number?`
+* `string?`
+* `boolean?`
+* `numeric?`
+
 ## Contributing
 
 1. Fork it ( https://github.com/soutaro/strong_json/fork )
 2. Create your feature branch (`git checkout -b my-new-feature`)
 3. Commit your changes (`git commit -am 'Add some feature'`)