[w3c/webpayments-methods-tokenization] Better orient the reader (#51)

It might be easier for people to implement the spec if we do a better job of orienting the reader about tokenization and how it fits into the payment ecosystem. This could be just a few sentences in the introduction and definition of some key terms (rather than pointing off to EMVCo documents, which themselves assume a fair amount of payment industry knowledge).

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https://github.com/w3c/webpayments-methods-tokenization/issues/51