Add NOTICE, and fix CC BY-SA 4.0 ambiguity

[thaJeztah]

• relates to LICENSE and LICENSE.doc causes go-license-detector to see only evidence of CC-BY-SA-4.0 #68 (comment)

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• Reset the Apache-2 License to be a pristine copy
• Add a NOTICE to mention copyright and licensing
• Update the README which referred to the docs license without "ShareAlike", but the license is CC BY-SA 4.0.

[thaJeztah]

This is the tricky part because of the ambiguity that was there; the original test described it without ShareAlike, but then used CC BY-SA further on; fcf056d

This README.md file and the CONTRIBUTING.md file are licensed under the Creative Commons Attribution 4.0 International License under the terms and conditions set forth in the file LICENSE.docs. You may obtain a duplicate copy of the same license, titled CC BY-SA 4.0, at http://creativecommons.org/licenses/by-sa/4.0/.

Given that the license itself is ShareAlike and referred to as CC BY-SA (as well as linked), I guess that's the most correct way to fix the ambiguity.

They're quite different though in some ways;

• CC BY-SA requires modified versions to keep using the same license (“share alike”).
• Plain CC BY only requires attribution and allows relicensing derivatives under different terms.

[thaJeztah]

Honestly, I wonder if it's worth the complexity to have these two files under a separate license, and if it would make more sense to just put them under the Apache-2 license.