It's in the errata and clarifications thread.
But actually, I'll re-post here and pin this to the top, so people can more easily see it (or at least, request it be done since I seem to have lost that ability):
Rules for thrown explosives ARE in the books, sort of, but you kind of have to interpret a few different rules to put together how they work. Not putting them together clearly was an oversight on my part. In the interest of clarity, here's how throwing a grenade, stick of dynamite, Molotov cocktail, or other hurled explosive works:
Make a basic ranged attack as a thrown weapon, targeting a spot rather than a person. Range increments are 10 feet, plus 5 feet per point of strength modifier, and the AC to hit is 15.
If your attack succeeds the grenade lands where you want it to go. If you miss, roll a d8 and a d4 for the misfire. The d8 represents the direction of the miss, with the 1 being short of your target and the 5 being an overthrow, moving clockwise from the 1 for direction. The d4 represents increments of five feet. Thus, if you roll a 1 on the d8 and a 2 on the d4, the grenade lands 10 feet closer to you than the target. If you were to roll a 2 on the d8, the grenade would be 10 feet off from center towards you and to the left. Yes, it's possible to drop the grenade behind you this way.
Damage is 6d6 to a 10 foot radius, ignoring armor. Dex save for half.