Similar to the existing 'recording-level-rels' and 'work-level-rels' in release queries it would sometimes be useful to also allow release-group-level-rels (e.g. when loading a release and one wants to know whether this release's release group is also part of a series).

The request URL could look like this:
beta.musicbrainz.org/ws/2/release/c646817e-bcc6-46a8-8d6b-945fcb2fc75f?inc=release-groups+release-group-level-rels+series-rels