[heycam/webidl] Import the Promises Guide. (#772)

Fixes #490.
You can view, comment on, or merge this pull request online at:

  https://github.com/heycam/webidl/pull/772

-- Commit Summary --

  * Import the Promises Guide.

-- File Changes --

    M index.bs (408)

-- Patch Links --

https://github.com/heycam/webidl/pull/772.patch
https://github.com/heycam/webidl/pull/772.diff

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