[Contributing] extended Symfony 2.4 maintenance

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

Maintenance for Symfony 2.4 has been extended from July 2014 to September 2014 (see [the blog post](http://symfony.com/blog/extended-maintenance-for-symfony-2-4)).