Generate the url path for the readme of an entity.

        @entity_id The id of the entity (i.e. charm, bundle).
        @param channel Optional channel name.

def entity_readme_url(self, entity_id, channel=None):
        '''Generate the url path for the readme of an entity.

        @entity_id The id of the entity (i.e. charm, bundle).
        @param channel Optional channel name.
        '''
        url = '{}/{}/readme'.format(self.url, _get_path(entity_id))
        return _add_channel(url, channel)