Add note about journal metadata block in User Guides appendix

[jggautier]

Users are told in the "Supported Metadata" section (http://guides.dataverse.org/en/4.7/user/dataset-management.html#supported-metadata) that for "more details about what Citation and Domain Specific Metadata is supported please see our Appendix", which points to the Metadata References page:

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Since a user could include a journal editor, the journal metadata block should be mentioned. Right now, I don't think the journal metadata block is mentioned anywhere in the guides.

Fields in the journal metadata block aren't mapped to any standards, so it doesn't make sense to list it with the citation and domain-specific blocks. Maybe a note at the bottom would work best:

Something like "Dataverse also has a journal metadata block, created for journals creating dataverses. See the .tsv version".

[pdurbin]

Thanks, for opening this issue, @jggautier . Once we add the Journal metadata block to the User Guide, we will have consistency with what curators and admins see when they go to a dataverse and click "Edit" then "General Information" and then scroll down to "Metadata Fields". Here's a screenshot: