Add angular.js style guide (#4621)

Add a style guide that helped me with angular.js

free-programming-books.md

@@ -1689,6 +1689,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Angular for the jQuery developer](http://www.ng-newsletter.com.s3-website-us-east-1.amazonaws.com/posts/angular-for-the-jquery-developer.html)
 * [Angular.js Guide](https://docs.angularjs.org/guide/)
 * [Angular.js Material Designing](https://material.angularjs.org/latest/)
+* [Angular.js Style Guide](https://github.com/toddmotto/angularjs-styleguide)
 * [Angular.js Tutorial](https://docs.angularjs.org/tutorial)
 * [AngularJS - Step by Logical Step](http://nicholasjohnson.com/angular-book/)
 * [AngularJS Notes for Professionals](https://books.goalkicker.com/AngularJSBook) (PDF)