Update README.md

README.md

 * [Cake Validation](https://github.com/cakephp/validation) - Another validation library (CP).
 * [Filterus](https://github.com/ircmaxell/filterus) - A simple PHP filtering library.
 * [ISO-codes](https://github.com/ronanguilloux/IsoCodes) - A library for validating inputs according standards from ISO, International Finance, Public Administrations, GS1, Book Industry, Phone numbers & Zipcodes for many countries.
+* [JSON Schema](https://github.com/justinrainbow/json-schema) - A [JSON Schema](http://json-schema.org) validation library.
 * [MetaYaml](https://github.com/romaricdrigon/MetaYaml) - A schema validation library that supports YAML, JSON and XML.
 * [Respect Validation](https://github.com/Respect/Validation) - A simple validation library.
 * [Upload](https://github.com/brandonsavage/Upload) - A library for handling file uploads and validation.