Added App::uses() and an explanation to the model defintion example

Added App::uses() to the code block and a brief explanation in order to avoid confusion with CakePHP's lazy loading, such as in the case where someone tries to make a static call and a normal class call has not yet been made.  This can occur whenever someone is creating a model from scratch and is using this doc for reference, and didn't use the Bake console to generate their model.