added comment to how many weeks variable var howManyWeeks

also fixed wording on prompt message for fourth week and on the how
many weeks variable prompt message. Changed the word "add" to "enter".
added giftCard variable and commented about the line
added comment to var giftCard variable line
added equation

added up the weekly song purchases, then multiplied the total to how
many weeks, and then subtracted that number by the amount of a gift
card.
workspace file added to idea folder