[w3c/browser-payment-api] Remove issue 55 note and add security considerations section. (#142)

Removing issue 55 note per [#55](https://github.com/w3c/browser-payment-api/issues/55#issuecomment-211796569). Adding initial starting point for security considerations section.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/browser-payment-api/pull/142

-- Commit Summary --

  * Remove issue 55 note and add security considerations section.

-- File Changes --

    M specs/paymentrequest.html (21)

-- Patch Links --

https://github.com/w3c/browser-payment-api/pull/142.patch
https://github.com/w3c/browser-payment-api/pull/142.diff

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