Do not add a heater. If worried throw some hay in there and they will make a cozy sleep area and stay warm. Most important is to keep them out of drafts but ventilate well. Most chickens die from frost bite and this is caused by moisture in the air and then freezing on the chickens combs and wattles. I read where one guy in the north, only closes the big sliding door to the coop if there is a blizzard but note that the doorway is facing the opposite of the wind direction.
Speaking of combs, it you have a breed that has a large single comb or a large standing comb (buttercup), you can rub a small amount of Vaseline on it to help keep it from freezing up.
Also make sure they have water and the water they have doesn't freeze up. There are lots of products on the market and if you search the web you can find homemade solutions too.
Some breeds lay through out winter but you can add a light too. However do not leave the light on 24 hours, they need to sleep and rest. I would suggest a timer for the light and turn on the mornings so they can go to bed with the setting sun.
I personally let my girls go all natural as in I don't add light. I let them go ahead and molt and take a break from egg laying, but I'm not selling eggs so don't need to keep the girls going.
