Woah! Dinnerbone is starting to tell us things! https://twitter.com/Dinnerbone/status/844544196744794112
That barely tells anything. The one below is way better.
The link in that tweet downloads a png file containing a 7-zip file with the password "advancements" containing two folders with two json files each. Someone on the subbreddit was kind enough to put them into a pastebin.