Add WebNN implementation status contribution mechanics

[ibelem]

Fix #66 .

@anssiko PTAL

[anssiko]

Thanks for this initial contribution! This looks great overall, added some suggestions.

[ibelem]

@anssiko Luckily I am still in the office and your feedback has been addressed and updated in the new commit, PTAL

[anssiko]

Thanks you the updates! Happy to merge this now.

To surface these guidelines more prominently, we can add a direct link at the bottom of https://webmachinelearning.github.io/webnn-status/ e.g.:

Interested in contributing to this implementation status page? See <a href="...">contributing guidelines</a>.