This might help. Look toward the bottom. Remember, just because you feed something on this list does not mean they will topple over dead immediately. A lot of the stuff is fairly safe in small amounts but can do internal, unseen damage, especially if it is fed often. Notice that green potato peels are on the list. Regular potato peels are fine, but the green ones contain solaline (?Spelling?) which is dangerous for humans as well as chickens.
I'll also mention cabbage. You'll see that discussed in some posts. It is on the list as a treat and many of us give them cabbbage or similar crops. I sure do. But if cabbage makes up a significant part of their diet, it contains a substance that can build up and harm them. It can harm you too if you eat too much, but it takes a lot. Feeding too much of anything can be bad for them, even if it is something that is good for them in proper amounts.