[w3c/ServiceWorker] Ask for web-platform-tests in CONTRIBUTING.md (#1131)

The wording is adapted from the WHATWG contributor guidelines:
https://github.com/whatwg/meta/blob/master/CONTRIBUTING.md

This is sometimes already happening:
https://github.com/w3c/web-platform-tests/pull/3449
https://github.com/w3c/web-platform-tests/pull/5628

Drive-by: whitespace
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1131

-- Commit Summary --

  * Ask for web-platform-tests in CONTRIBUTING.md

-- File Changes --

    M CONTRIBUTING.md (12)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1131.patch
https://github.com/w3c/ServiceWorker/pull/1131.diff

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