code: public BoardingPassBuilder addAuxiliaryField(String label, String value) {
  		Field field = new Field(label, value);
  		this.boardingPass.addAuxiliaryField(field);
  		return this;
  	}
docs: Adds an auxiliary field for the current {@link BoardingPass} object. This
  field is optional. There can be at most 5 auxiliary fields per boarding
  pass.
  
  @param label
  the label for the additional field. It can't be empty.
  @param value
  the value for the additional field. It can't be empty.
  @return this builder.