Added a message about XML mapping in the doc
Added a message about XML mapping in the doc

stof/StofDoctrineExtensionsBundle,IWAtech/StofDoctrineExtensionsBundle,alterphp/StofDoctrineExtensionsBundle,felicitus/StofDoctrineExtensionsBundle,romaricdrigon/StofDoctrineExtensionsBundle,mcfedr/StofDoctrineExtensionsBundle,jhcasado/StofDoctrineExtensionsBundle,erikaheidi/StofDoctrineExtensionsBundle,OskarStark/StofDoctrineExtensionsBundle