[w3c/webcomponents] Add Scoped CustomElementRegsitry explainer (#865)

See issue #716 

I've tried to incorporate feedback from the issue, last F2F, and reviews from colleagues.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/865

-- Commit Summary --

  * Add Scoped CustomElementRegsitry explainer

-- File Changes --

    A proposals/Scoped-Custom-Element-Registries.md (227)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/865.patch
https://github.com/w3c/webcomponents/pull/865.diff

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