Update .gitignore for Symfony2

Updates `Symfony2.gitignore` to reflect changes to [Symfony's `.gitignore`](https://github.com/symfony/symfony-standard/blob/master/.gitignore).

Changes include adding the `bin` folder to the list of ignored folders and adding an exception for `app/cache/.gitkeep` and `app/logs/.gitkeep`.

Also reordered the entries and improved the documentation so it makes more sense, especially for new users of Symfony.