Although Aunt Sally puts stress on separately feeding oyster shell in actual fact you don't needed to as if you feed them good quality layers (pellets preferably) this contains a balanced diet for egg laying and has all the calcium they need. You do need to ensure they have access to insoluable grit as hens don't have teeth and they rely on grit in their gizzards to grind up their food (particularly if they eat corn). Hens kept on grass with access to open ground will find all the grit they need unless it's clay and therefore to be sure you can suppliment it with mixed grit which contains oyster shell as well as insoluable grit. (Grit comes in soluable (generally oyster shell) or insoluable (crushed gravel) or mixed). I use mixed in a sepaqrte gritbox not in their food, but really it's just extra insurance.
Re nutrition generally you should feed a basic diet of layers pellets. Much easier for a small number of hens than mash which needs to be fed in troughs per meal whereas pellets are more hygenic in free range systems and can be fed ad lib in gravity hoppers. You can suppliment this with a treat such as a handful of corn per hen each evening (no more). Household scraps are OK but don't over feed or you'll unbalance their diet.
Chickens are like children who would eat crisps, ice cream and sweets permanently if you offered them. It's OK to offer them "treats " ocassionally (once a week and no more than 10% of their total diet) things such as sweet corn, pasta, cooked rice. Fresh veg such as lettuce tomatoes, grapes, apples, cabbage is Ok. Mine don't touch carrot. Potatoes & peelings should be cooked as starch is indigestible. They will also eat most things with seeds such as melon seeds or seeds from peppers though not the chilli ones (at least I haven't tried this). Don't give them citrus fruit at all.
Some people on this site feed cat meat as an extra protein suppliment and a treat but I'm very dubious about this as overweight hens are unhealthy and they can be prone to being obese quite apart from the fact it's illegal to do this if there's any danger of the animal entering the human or animal food chain. If you feel the need to provide extra protein for a medical issue such as recovery from a heavy moult or illness you can feed growers pellets which contain more protein or I guess dried cat food if you don't intend to eat the chicken. However once the need has passed revert back to layers which as I have said provides a healthy balanced diet. You'll find lots of people advocating alsorts of concoctions to feed then . I've seen people who think it's a good idea to mix rolled oats, honey & yogurt and other goodies together as their patent health mix. They maybe OK occassionallly as a treat but for normal staple diet trust the experts at the feed companies who have proven their formula scientifically and don't be tempted to overdose on suppliments unless your vet or a similarly knowledgible person has recommended it for a medical reason.
Incidentally I use Spillers Layer pellets, poultry corn, and Marriages Growers.
Regards