Add .gitignore file for Symfony3

Although version 3 of the Symfony Standard Distribution has not yet been
released, version 2.5 already offers the possibility to start using the
new directory structure that will come with version 3. The new directory
structure introduces a new 'var' folder (containing cache and log files
and other files that are changed by Symfony itself) and some other minor
changes.

This new file, Symfony3.gitignore, can be used for Symfony applications
that use this new directory structure.