[webcomponents] Add "why upgrades" document to proposals section (#112)

This hopes to explain why upgrades are important.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/112

-- Commit Summary --

  * Add "why upgrades" document.

-- File Changes --

    A proposals/Why-Upgrades.md (125)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/112.patch
https://github.com/w3c/webcomponents/pull/112.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/pull/112