Add Chronos DateTime library. (#677)

* Add Chonons DateTime library.

* Fix typo.

README.md

 
 * [CalendR](http://yohan.giarel.li/CalendR/) - A calendar management library.
 * [Carbon](https://github.com/briannesbitt/Carbon) - A simple DateTime API extension.
+* [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.