He got caught cheating and was forfeited.
Wesley So resigned after 6 moves

Yes, he didn't resign. He was forfeited.
"Cheating" is too harsh of a word, I think, and certainly not very descriptive.
He was guilty of a writing down notes during the game, and had been warned at least once, possibly twice, before. Yasser said the notes had nothing to do with the game, but that is irrelevant. The rules say you can't take notes!
He has the opportunity to appeal.
Will this game change his rating? I think forfeited games do not affect ratings, but I'm not sure about cases like this one.

Will this game change his rating? I think forfeited games do not affect ratings, but I'm not sure about cases like this one.
I think it counts since moves were played.

eh. If it had nothing to do with chess it could've been some type of family/friend emergency that he needed to be made aware of. in that case, I would tell chess to ef itself.
But really other than that-and not cheating- what the heck would it be? why risk something so pointless (unrelated unimportant notes) over something so important (championship), even after being given a warning. Something doesn't make sense.

All it has to be is a code that he understands that are notes about his prep, his analysis during the game, etc.
It's cheating. PERIOD.

If the notes were something like,
"Reminder: Don't get into time trouble!" or
"Reminder: Take a look at this variation in more detail after the game." or
"Don't forget to wish my G'ma a happy birthday before tomorrow."
then one can argue that those types of notes aren't cheating. Of course, "cheating" can mean many things, of varying degrees.
But any kind of notes are against the rules and he was warned at least twice it's now been said, hence the forfeit.

Apparently he was writing on a separate piece of paper this time, thinking that it was only the actual scoresheet that he wasn't allowed to write on.
I can't imagine he can have any grounds for appeal though. I don't think at all that he was cheating, but rules are rules and when you have been warned twice (which Alejandro Ramirez said is a courtesy - the arbiter can simply disqualify you the first time without a warning) there can't really be any argument.
If nothing else it is a distraction for your opponent.
Cool move