Data models in Payment Method Specs

Hi all,

This may have been obvious to you all but it was not to me so I want to
point it out and make sure we document it for others.

When writing a payment method spec it's important to define the input and
output data models using a dictionary (not an interface) and I avoid
defining custom interfaces altogether. As Marcos explains in this issue [1]
a dictionary, when constructed and consumed by the website is just a
regular Javascript object so there is no need to look for special types.

Adrian

[1] https://github.com/w3c/browser-payment-api/issues/526