See https://bugs.kde.org/show_bug.cgi?id=393106 .
Currently, all music files are included if their metadata are complete enough to allow their use. This feature is about filtering the results for music in the Baloo index by their path to only include files matching the folders configured in Elisa configuration dialog.
The main needed skill is c++ . You would get support by the Elisa team. this feature is only requiring modifications into the few classes adding support for Baloo.