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
