Our JSON dumps contain both CC0 and supplementary CC BY-NC-SA data (annotations, tags and genres, ratings, at least). But both the docs and the dumps themselves claim the whole thing is CC0, which is not true.

We should:

• Update the documentation (https://musicbrainz.org/doc/Development/JSON_Data_Dumps) to specify which parts are CC0 and which are CC BY-NC-SA
• Also add the COPYING-CCShareAlike file to the dump alongside COPYING-PublicDomain
• Add info to the README inside the dump to specify which parts are CC0 and which are CC BY-NC-SA