How can I know when new iOS updates are released?
ipsw.me provides an RSS feed of iOS releases at ipsw.me/timeline.rss. You can subscribe to this RSS feed (or a filtered version) using IFTTT. Templates are provided at ipsw.me/notifications. This is accurate to a couple of minutes.

You can also subscribe to the TSS status of iOS releases using the API available at api.ineal.me. This can be near instantaneous, providing a real-time view of TSS signing (and subsequently the releases).