Added most important pieces of metadata for publication

This fixes what's more important about #106.
See [Returning metadata about the document](https://github.com/w3c/specberus/blob/tripu/issue-106-metadata/README.md#returning-metadata-about-the-document) for more information about the way metadata is returned.
Since there can be multiple groups, I would s/group/groups/. I might be wrong, but I think the reason why it's called `deliverers` in the existing system rather than `groups` is because entities can publish, like Task Forces. If I am wrong and TFs cannot publish, then I don't know why it was obscured in the DB.