Add Recurr library to date handling section

README.md

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