update readme to clarfiy how to update the GTFS feed

[elad661]

Calling update_feed.sh when you already have the database won't work. The way I've been doing it on the server, to save time, is having two databases, gtfs and gtfs2.

When it's time to update the GTFS feed, I modify the config to point to the other database, run update_feed.sh, and then when it's done - restart curlbus. This way curlbus can continue working while the feed is updated, there's no need for complicated GTFS feed update logic, and I can revert to the older GTFS copy in an atomic operation.

I should update the README to reflect that, and also write a script to automatically do the update&switch.