[paymentrequest] Add rationale and update document locations (#62)

Added a rationale document as an addition to original explainer. Moved these into docs/.
You can view, comment on, or merge this pull request online at:

  https://github.com/WICG/paymentrequest/pull/62

-- Commit Summary --

  * Add rationale and reconfigure repo structure to make more sense by having explicit doc loation
  * Fix links and update text
  * Typing is hard - fix file names
  * Updates based on feedback
  * Broke images again - oops
  * Make location change known

-- File Changes --

    M README.md (6)
    A docs/explainer.md (236)
    R docs/images/flow_diagram.png (0)
    R docs/images/flow_diagram.svg (0)
    A docs/images/new-checkout-flow.png (0)
    A docs/images/standard-checkout-flow.png (0)
    A docs/rationale.md (84)
    M explainer.md (237)

-- Patch Links --

https://github.com/WICG/paymentrequest/pull/62.patch
https://github.com/WICG/paymentrequest/pull/62.diff

---
Reply to this email directly or view it on GitHub:
https://github.com/WICG/paymentrequest/pull/62