code: func DigestFile(filename string) (string, error) {
  	f, err := os.Open(filename)
  	if err != nil {
  		return "", err
  	}
  	defer f.Close()
  	return Digest(f)
  }
docs: // DigestFile calculates a SHA256 hash (like Docker) for a given file.
  //
  // It takes the path to the archive file, and returns a string representation of
  // the SHA256 sum.
  //
  // The intended use of this function is to generate a sum of a chart TGZ file.