code: func AddFile(a Archive, file string) error {
  	fd, err := os.Open(file)
  	if err != nil {
  		return err
  	}
  	defer fd.Close()
  	fi, err := fd.Stat()
  	if err != nil {
  		return err
  	}
  	w, err := a.Header(fi)
  	if err != nil {
  		return err
  	}
  	if _, err := io.Copy(w, fd); err != nil {
  		return err
  	}
  	return nil
  }
docs: // AddFile appends an existing file to an archive.