Scoring for the interop dashboard

[stof]

It would be great to publicly state the scoring criteria for this investigation area (for reference, see web-platform-tests/interop-accessibility#148 for the accessibility investigation area) to give visibility over this investigation area.
Right now, the dashboard reports it at 0%.

[stof]

I see that 7302a3c has updated the readme with some progress status.
However, this update still does not produce a way to compute a single score, as expected by the interop dashboard.