[w3c/webcomponents] Add css-modules-explainer-v1.md (#827)

Add CSS explainer.  Moved from https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/master/CSSModules/v1Explainer.md.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/827

-- Commit Summary --

  * Add css-modules-explainer-v1.md

-- File Changes --

    A proposals/css-modules-v1-explainer.md (156)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/827.patch
https://github.com/w3c/webcomponents/pull/827.diff

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https://github.com/w3c/webcomponents/pull/827