Merge pull request #713 from itcraftsmanpl/php-ml

Add link to php-ml

README.md

 * [PHP Expression](https://github.com/Kitano/php-expression) - A PHP expression language.
 * [PHP PassBook](https://github.com/eymengunay/php-passbook) - A PHP library for iOS PassBook.
 * [PHP-GPIO](https://github.com/ronanguilloux/php-gpio) - A library for playing with the Raspberry PI's GPIO pins.
+* [PHP-ML](https://github.com/php-ai/php-ml) - A library for Machine Learning in PHP.  
 * [PHPCR](https://github.com/phpcr/phpcr) - A PHP port of the Java Content Repository (JCR).
 * [PHPStack](http://dunkels.com/adam/phpstack/) - A TCP/IP stack proof of concept written in PHP.
 * [print_o](https://github.com/koriym/print_o) - An object graph visualizer.