paddle: add note about maintenance status
all: update license text and filename

The zip files used by Go Modules use case-insensitive file names so
LICENSE collides with license/. Add a .txt extension to fix this.
all: change import path to code.soquee.net
all: add a license file
all: add readme mentioning maintenance status

This project was an experiment but I didn't end up using Paddle, so this
library will no longer be maintained. Just in case anyone ever finds it
and decides to use it for some reason, make the maintenance status clear
in a readme file.
all: normalize package and Client docs
payment: new package with payment APIs
user: new package with user APIs
paddle: improve NewRoundTripper docs
plan: new package with plan APIs
paddle: add other supported currencies
coupon: remove stutter in delete method
paddle, coupons: add new Currency type
paddle, license: move license code to new package
paddle, product: move product code to new package
paddle: use consts from internal
paddle, coupons: move coupon code to new package
internal: new package with shared consts
paddle: initial handful of API endpoints