Ask for web-platform-tests in CONTRIBUTING.md

[foolip]

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

This is sometimes already happening:
web-platform-tests/wpt#3449
web-platform-tests/wpt#5628

Drive-by: whitespace

[foolip]

Discussed briedly with @jakearchibald before sending, this is a starting point for discussion, as what worked for HTML might not work verbatim for Service Workers. Especially interested in feedback from @jungkees.

[jungkees]

Basically, I agree to do this. Service workers folks already agreed on the importance of the tests during the f2f. I think we can allow some wiggle room where providing tests is not practical as the PR suggests. Any other comments?

[foolip]

That's great, thanks @jungkees! Like in whatwg/html#1849, you of course need not assume that it's going to work out perfectly, and you can change the "rules" as needed to work well for Service Workers.

[jungkees]

@foolip, it seems like we also need whatwg/meta@198c127?

[foolip]

Good idea. I tried to integrate that while keeping it to a single paragraph, PTAL.

(I think that if many specs do this, it might be best to extract out "process" to e.g. a page on web-platform-tests.org, but I'm not in a hurry to generalize.)

[jungkees]

LGTM. Merged. Thanks. Curious how the other specs will do for it.

[foolip]

I'm trying to figure out which specs are most ready to adopt this working mode, if you know of others where you think it'd make sense, please let me know!