README.md: Add authoritative_source and distribution_scope labels, ad…

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…d Details sections

Adding authoritative_source and distribution_scope labels to allow locating the original source of an image as well as encoding the intended scope of distribution.

Adding a section with details on the label fields to add context and discuss details.

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Once this is merged, I will add details for other labels.

[rhatdan]

@riekrh squash your pulls.

git rebase -i origin Take second patch and change pick to f Save git push riekrh --force

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@rhatdan done

[ghost]

@jonboulle does this work for you?

[rhatdan]

No protests, so LGTM