Provide Integration Instructions for Starter Frontend

[jarednorman]

I installed this on a new store using the starter frontend and found that out of the box the promotions I was creating weren't working when I put the coupon code in on the cart page. This was because the CouponCodesController references the stock Solidus::PromotionHandler::Coupon. Swapping it out for SolidusFriendlyPromotions::PromotionHandler::Coupon fixes the issue.

I'm not sure if there are other touchpoints that need to be handled, but we should probably document at least this touchpoint.

[mamhoff]

I'll get to that shortly, for now there's a PR to solidus_starter_frontend to make this not an issue by default for new stores.