Ask for tests for normative changes in CONTRIBUTING.md

[rakuco]

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.

[rakuco]

@sgraham @luser @toji

[rakuco]

Most gamepad tests in w-p-t are currently manual, but I still think it makes sense to move forward with this and work on better, automated tests if necessary.

[rakuco]

@sgraham @luser @toji: friendly ping

[sgraham]

Thanks