Ask for tests for normative changes in README

[foolip]

See https://github.com/foolip/testing-in-standards/blob/master/policy.md
for context. This phrasing matches that used for many other specs'
CONTRIBUTING.md files, in particular:
https://github.com/w3c/fxtf-drafts/blob/master/CONTRIBUTING.md#tests
https://github.com/w3c/csswg-drafts/blob/master/CONTRIBUTING.md#tests (slightly different)

[foolip]

Hi Houdini folks! Take this PR as a humble suggestion, I haven't talked to anyone about it in advance, but wanted to get the idea out there for discussion.

Here more than in other domains I think the tentative tests work flow described in https://github.com/foolip/testing-in-standards/blob/master/lifecycle.md could make sense.

[tabatkins]

I'm 👍 for it.

[foolip]

Is there a meeting or something where this needs to be discussed, or what are the next steps? To what extent is this (tests with changes) already happening?

[foolip]

Can this be put on the agenda for TPAC, if there is one?

[foolip]

Well, TPAC is long gone. @tabatkins, anyone else I should poke for a go/no-go, or just close this issue? Tests are being written for at least CSS Typed OM, so that's nice.

[nainar]

@atanassov and @shans since you are both chairs how does this PR look to you?

Personal note: Big +1!