Add Fractal.

README.md

 * [PHP Collections](https://github.com/schmittjoh/php-collection) - A simple collections library.
 * [Serializer](https://github.com/schmittjoh/serializer) - A library for serialising and de-serialising data.
 * [PHP Object Storage](https://github.com/herrera-io/php-object-storage) - A library for object storage.
+* [Fractal](https://github.com/php-loep/fractal) - A library for converting complex data structures to JSON output.
 
 ## Notifications
 *Libraries for working with notification software.*