Get the readme for an entity.

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

def entity_readme_content(self, entity_id, channel=None):
        '''Get the readme for an entity.

        @entity_id The id of the entity (i.e. charm, bundle).
        @param channel Optional channel name.
        '''
        readme_url = self.entity_readme_url(entity_id, channel=channel)
        response = self._get(readme_url)
        return response.text