code: func (b *Builder) ReadFile(path string) (Source, error) {
  	data, err := ioutil.ReadFile(path)
  	if err != nil {
  		return Source{}, fmt.Errorf("config: ReadFile failed on %s: %s", path, err)
  	}
  	return Source{Name: path, Data: string(data)}, nil
  }
docs: // ReadFile parses a JSON or HCL config file and appends it to the list of
  // config sources.