add note to docs explaining grand total must be greater than zero
add note to docs explaining grand total must be greater than zero

fbernardo/stripe-ios,NewAmsterdamLabs/stripe-ios,stripe/stripe-ios,fbernardo/stripe-ios,stripe/stripe-ios,fbernardo/stripe-ios,stripe/stripe-ios,fbernardo/stripe-ios,NewAmsterdamLabs/stripe-ios,stripe/stripe-ios,NewAmsterdamLabs/stripe-ios,NewAmsterdamLabs/stripe-ios,stripe/stripe-ios,stripe/stripe-ios