code: func (f *FileResourceLookup) Lookup(file, relativeTo string) ([]byte, string, error) {
  	file = relativePath(file, relativeTo)
  	logrus.Debugf("Reading file %s", file)
  	bytes, err := ioutil.ReadFile(file)
  	return bytes, file, err
  }
docs: // Lookup returns the content and the actual filename of the file that is "built" using the
  // specified file and relativeTo string. file and relativeTo are supposed to be file path.
  // If file starts with a slash ('/'), it tries to load it, otherwise it will build a
  // filename using the folder part of relativeTo joined with file.