added section about how to correctly add spec links in tests

@plinss - In an effort to clarify this ever present confusion, I'm adding a note about linking specs here. I chose to put this info here over the CSSWG wiki as I think it will be more easily discoverable.  Also, kept it off testthewebforward.org since it's specific to our WG.  Did I correctly document this?

Once this gets merged, I plan on updating the validation code to refer people here so they will be better able to fix the errors they now see on the automatic validation.