docs: def add_extension(self, extension):
          """
          Specify a broadway extension to initialise
  
          .. code-block:: python
  
              factory = Factory()
  
              factory.add_extension('broadway_sqlalchemy')
  
          :param extension: import path to extension
          :type extension: str
          """
          instance = werkzeug.utils.import_string(extension)
  
          if hasattr(instance, 'register'):
              instance.register(self)
  
          self._extensions.append(instance)