Jeremy Crawford: The Ready action lets you ready any action you can normally take, including Attack enhanced by Extra Attack.
I think this makes a lot of sense - it's like changing intiative order without the fuss.
He made a goof there, Extra Attack specifically says it only works on your turn. Or maybe he's giving RAI.
That's what I mean by changing the initiative order without the fuss - readying makes a later turn 'your turn' in the same round. Perhaps 'your turn' is split over the ready part and the acting part? He doesn't say 'I would rule...' or anything that would indicate RAI.
Well, either he made a mistake about how Extra Attack works, he's offering errata for Extra attack, or he's errataing the Ready rules. Guess you can take it any way you like, but to me Occam's razor suggests the first. 
That's a pretty far stretch for RAI when extra attack actually specifically says on your turn... Guessing it's a rare mistake on his part.
Is the tweet deleted? Link is broken now.
El_Condoro wrote:
Ah he caught himself
We all make mistakes
https://www.youtube.com/watch?v=F-VWeFoPxvM
Ha that was quick.