Add details of export fields to readme.
Add details of what the fields within an export relate to.
Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/feedback/-/issues/11.
Add details of what the fields within an export relate to.
Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/feedback/-/issues/11.