public Builder addSourceFolder(@NonNull File folder){
  foldersToScan.add(folder);
  return this;
}

public Builder addSourceFolder ( @ NonNull File folder ) { foldersToScan . add ( folder ) ; return this ; }
root folder for labels - > documents . each subfolder name will be presented as label , and contents of this folder will be represented as labelleddocument , with label attached
