docs: Add license: Creative Commons Attribution 4.0 International License

Not sure this has ever discussed but I'd like to proposse to use this license for the docs (seems they don't have any at the moment)

If it helps deciding, note flatpak uses this conbination of licenses as well:

• https://github.com/flatpak/flatpak/blob/master/COPYING
• http://docs.flatpak.org/en/latest/