Update installation.rst
Update installation.rst

Add undocumented configuration for defining custom classes instead of using the default Application/Sonata/MediaBundle
hug963/SonataMediaBundle,nicolasricci/SonataMediaBundle,jerome-fix/SonataMediaBundle,hug963/SonataMediaBundle,rmzamora/SonataMediaBundle,nicolasricci/SonataMediaBundle,hug963/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,nicolasricci/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,hug963/SonataMediaBundle,bgaleotti/SonataMediaBundle,OskarStark/SonataMediaBundle,nicolasricci/SonataMediaBundle,vyshkant/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,jerome-fix/SonataMediaBundle,sonata-project/SonataMediaBundle,OskarStark/SonataMediaBundle,rmzamora/SonataMediaBundle,peterkokot/SonataMediaBundle,Soullivaneuh/SonataMediaBundle,vyshkant/SonataMediaBundle,vyshkant/SonataMediaBundle,bgaleotti/SonataMediaBundle,bgaleotti/SonataMediaBundle,OskarStark/SonataMediaBundle,peterkokot/SonataMediaBundle,jerome-fix/SonataMediaBundle,vyshkant/SonataMediaBundle,OskarStark/SonataMediaBundle,rmzamora/SonataMediaBundle,vyshkant/SonataMediaBundle,jerome-fix/SonataMediaBundle,hug963/SonataMediaBundle,peterkokot/SonataMediaBundle,bgaleotti/SonataMediaBundle,nicolasricci/SonataMediaBundle