Add copyright note

[jo-mueller]

@dstansby rightfully pointed out that the original spec template contained a license note for derivative work. I would tend to say that the spec, at this point, doesn't have to do much with the original text anymore, so maybe adding the note would suffice?

Otherwise, I am afraid that adding a BSD3 license here may be a bit of difficult/problematic thing to do, atm 😬

Edit: Did some more searching yesterday and I think doing it like this should be ok. Two of the original license statements don't apply:

• There is no software here that would have to be licensed under Apache 2.0
• There is no contribution to w3c content that would require consent to w3c CLA.

So we would only need to retain the copyright notice, which only applies to the parts of the template that are left over (which aren't many).

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