If a player readies the attack action and has extra attack, do they hit once?

@JTtheBeardless @JeremyECrawfordIf a player readies the attack action and has extra attack, do they hit once, or does extra attack activate? hit once – extra attack specifies it works when you use the Attack action on your turn

— (((Mike Mearls))) (@mikemearls) July 21, 2016