Update documentation about licences (CC-0 is also a valid licence choice)

[zed-g]

Since we (at correspSearch) just got an inquiry about licence options, I would propose to update the relevant parts of https://github.com/TEI-Correspondence-SIG/CMIF/blob/main/doc/documentation-en.md#information-about-the-digital-index-of-letters to also include the licence option of CC-0. To my knowledge this is also an accepted licence choice for CMIF data and we already process and store CMIF data with this licence for correspSearch. It is also available as a licence option for our CMIF Creator.

I can provide the necessary changes to doc/documentation-en.xml (but I have no clue about your conversion-to-markdown routine).

[peterstadler]

Makes sense to me to also allow everything looser (or equivalent) than CC-BY-4.0.

[rettinghaus]

In general the documentation reflects what correspSearch needs, but not the customization (e.g., licence can be empty and its target attribute can point anywhere). It would be good if the documentation differentiated between the plain schema and application needs.

[zed-g]

In general the documentation reflects what correspSearch needs, but not the customization (e.g., licence can be empty and its target attribute can point anywhere). It would be good if the documentation differentiated between the plain schema and application needs.

That's a good point, it's important to distinguish these two cases and be explicit about it. Either way, right now the documentation differs from the actual schema (and only mentions correspSearch once as far as I can see). If we leave the schema as is (with free choice of licences), we should at least strongly suggest/recommend CC-BY 4.0/ CC0/anything "looser" in the documentation I guess?