Document how to add entities

[laidig]

I need to add entities (specifically, a feed_info where one is not present in the original feed) to my feed. I've figured out how to do this, but it's not documented and may be a little convoluted:

if 'feed_info' not in gtfs_feed.by_id:
    gtfs_feed.by_id['feed_info'] = {}
    cls = gtfs_feed.FACTORIES['feed_info']
    info = cls.from_row({
        'feed_publisher_name' : agency_id,
        'feed_publisher_url' : url ,
        'feed_lang' : feed_lang,
        'feed_id' : agency_id
        })
    gtfs_feed.by_id['feed_info']['a'] = info

Is there a better way? If this is acceptable I will submit a PR explaining the process.