Doesn't exhaust to attack, but can only attack once per turn.
Readiness doesn't let you ignore any other attacking rules. You still can't attack with something that's exhausted, and you still can't attack with it the turn it comes under your control (unless it also has haste.) — Sirlin, 03/14/16
If you attack with something that has readiness, it won't exhaust. You CAN exhaust it to pay for the cost of a spell or ability (or as an effect from a spell or ability), even though you can't attack with it again that turn. — Sirlin, 03/14/16
Things with readiness can only attack once per turn, even if an effect would normally let them attack again. When cards leave play and then come back (for example, from Geiger or Pasternaak's max level abilities), they count as new objects though. So it IS possible to attack with something with readiness, have it leave play from one of those abilities, return, and if it has haste it could attack again. — Sirlin, 03/19/16