Merge pull request #743 from MatthiasKuehneEllerhold/add-yasumi

Add yasumi

README.md

 * [Chronos](https://github.com/cakephp/chronos) - A DateTime API extension supporting both mutable and immutable date/time.
 * [ExpressiveDate](https://github.com/jasonlewis/expressive-date) - Another DateTime API extension.
 * [Moment.php](https://github.com/fightbulc/moment.php) - Moment.js inspired PHP DateTime handler with i18n support.
+* [Yasumi](https://github.com/azuyalabs/yasumi) - An library to help you calculate the dates and names of holidays.
 
 ## Event
 *Libraries that are event-driven or implement non-blocking event loops.*