Update README.md

Additions:

* A Practical Guide to AngularJS Directives: [Part 1](http://www.sitepoint.com/practical-guide-angularjs-directives/), [Part 2](http://www.sitepoint.com/practical-guide-angularjs-directives-part-two/)
* [AngularJS Form Validation] (http://scotch.io/tutorials/javascript/angularjs-form-validation)
* AngularJS Tutorial by Jakob Jenkov: [Introduction](http://tutorials.jenkov.com/angularjs/index.html), [Views & Directives](http://tutorials.jenkov.com/angularjs/views-and-directives.html), [Events](http://tutorials.jenkov.com/angularjs/events.html), [AJAX](http://tutorials.jenkov.com/angularjs/ajax.html)
* [Leveraging Directives To Reduce $Watch'ers In Angular] (http://frontendandvodka.com/watchers-when-performance-is-key/)