Added MetaYaml

MetaYaml is a schema validation library I wrote some time ago (pretty mature IMO), I still maintain it and will publish probably a new version some time soon :)

README.md

 * [Valitron](https://github.com/vlucas/valitron) - Another validation library.
 * [Upload](https://github.com/codeguy/Upload) - A library for handling file uploads and validation.
 * [DMS Filter](https://github.com/rdohms/DMS-Filter) - An annotation filtering library.
+* [MetaYaml](https://github.com/romaricdrigon/MetaYaml) - A schema validation library, supporting YAML, JSON and XML.
 
 ## REST and API
 *Libraries and web tools for developing REST-ful APIs.*