I have added 17 basic Achievements, each one for defeating a level. So for instance if you now complete the first level, Alien Suburbia, you will receive an Achievement called Close Encounters - Completed E1M1. I have tested a bunch of levels and it seems to work fine, but I think you have to be online and Steam needs to have a connection to the internet for it to work. That's what it looks like at least.
I decided to keep the Achievements normal, not requiring extreme feats to unlock them, as I personally don't like it when they are locked behind insane difficulty or challenges. If you complete CARNAL, you completed it in every aspect. Hope that's okay, life is hard enough as is ;)
Please let me know if it doesn't work for you, and I'll see if I can somehow fix it, but so far it works on my end.