Write some documentation for how folks should handle saving relationships in the short-term

Eventually, we're going to have a way of adding related models and persisting that to the database. However, right now that jank don't exist. So, what we're left with is creating the related model and populating the foreign key by-hand. 

Which... isn't too bad really. I kinda like being explicit with it and a lot of times the interface to persisting relationships in ORMs is really weird. 🤷 