Anyone that knew anything about DC knew it was happening. Honestly, I'm not understanding why this is a big deal.
I knew he was in it and I also knew the trinity would team up at some point. I guess a bunch of people didn't know Doomsday would make an appearance.
At some point before the films release the fact that Doomsday makes an appearance would become public knowledge. At that point it doesn't take much to piece together what will happen when a bunch of heroes meets a villain.