Add cookbook entry about uploading and validating multiple files

[Koc]

http://symfony.com/doc/current/cookbook/controller/upload_file.html missing info about option multiple=true
http://blog.arithm.com/2014/11/24/validating-multiple-files-in-symfony-2-5/

[OskarStark]

👍

[javiereguiluz]

Here it is another recent article about this topic: http://www.theodo.fr/blog/2015/07/manage-multiple-files-upload-in-symfony/

[OskarStark]

Here it is another recent article about this topic:
http://www.theodo.fr/blog/2015/07/manage-multiple-files-upload-in-symfony/

Great article

[javiereguiluz]

Closing this in favor of the file uploading meta-issue: #8255.