I am using version 1.3.8 and am interested in adding a field to my checkout/billing page. I'd like to add a "where did you hear about us" field so I can offer referral discounts/incentives. If possible, i'd like to make this a "required" field. how would I go about doing this? I have no idea where the file is located that I need, but it's where there is a box for special instructions, plus the billing address and customer info.