There's my post here: https://hive.blog/hive-139531/@blocktrades/a-developer-s-introduction-to-the-hive-application-framework
I've also had them do some work on two existing HAF apps in the Hive repo (balance_tracker and HAfAH). And the UI guy has been doing some reading and experiments with the existing Hive API (documented here: https://developers.hive.io/).