[w3c/IndexedDB] Ask for web-platform-tests in CONTRIBUTING.md (#200)

The wording is adapted from the following documents:
https://github.com/whatwg/meta/blob/master/CONTRIBUTING.md#tests
https://github.com/whatwg/meta/blob/master/TEAM.md
https://github.com/w3c/ServiceWorker/blob/master/CONTRIBUTING.md
https://github.com/w3c/web-performance/blob/gh-pages/CONTRIBUTING.md#test-driven

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

  https://github.com/w3c/IndexedDB/pull/200

-- Commit Summary --

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

-- File Changes --

    M CONTRIBUTING.md (19)

-- Patch Links --

https://github.com/w3c/IndexedDB/pull/200.patch
https://github.com/w3c/IndexedDB/pull/200.diff

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