Document that the Braintree account must be configured to handle currencies

You have to manually set up each currency in Braintree that you want to use in Commerce. If you don't, then Braintree will ignore the Commerce currency configuration and charge the customer based on the Braintree account's default currency.

This is a Braintree configuration issue, not a Commerce issue, but since it can result in customers getting overcharged, I think it would be good to note this in the README. Patch incoming.