Adding the testharness.js subrepo to the csswg-test repo

Hey Peter,

I just updated our github instructions with info about the testharness (resources) subrepo [1].  Rather than writing up separate instructions for CSS test writers, can we just add it as subrepo as Tobie suggested in the PR comments?

Will that interfere with the other repo changes you're making?  Again, I'm concerned about the timing here because I'd like to have the repos and the docs all in place for the event -- preferably well before as we're encouraging registrants to get set up prior to that day.

Thanks!
-Rebecca

[1] https://github.com/w3c/testtwf-website/pull/162