docs: def berksfile(self):
          """Return this cookbook's Berksfile instance."""
          self.berks_path = os.path.join(self.path, 'Berksfile')
          if not self._berksfile:
              if not os.path.isfile(self.berks_path):
                  raise ValueError("No Berksfile found at %s"
                                   % self.berks_path)
              self._berksfile = Berksfile(open(self.berks_path, 'r+'))
          return self._berksfile