DATA dictionary: Add guide for working with unjoined Fulcrum .csv files

We need a guide to the raw data frames somewhere. Some sort of visual guid to how the data are joined would be helpful. For example, how does fulcrum_parent_id, fulcrum_child_id, fulcrum_id work in all the .csv files. This need to be made clear so the user knows how best to work with unjoined data.