add rules for new CakePHP 3

This is a demand for [CakePHP framework](http://cakephp.org)
The skeleton of a CakePHP 3 application can be found in [CakePHP docs](http://book.cakephp.org/3.0/en/installation.html#production)

The .gitignore file can be found directly in the project:
- CakePHP 3 : https://github.com/cakephp/app/blob/master/.gitignore

As discussed on github [here](https://github.com/cakephp/docs/issues/2027#issuecomment-63729955), we would like to add the .gitignore rules for the new CakePHP 3 version.

Thanks
I'll keep only C uppercase.
I'm wondering  if we can just keep the following for Cake2? All config files are in `app`?

    tmp/*
    app/tmp/*
    app/Config/core.php
    app/Config/database.php
    !empty