Documentation page/section in the model

I would like to have a place to do a more comprehensive description of what things were done they way they were in the model, as well as a list of things to complete, technical debt to clear up, etc. Documentation in other words, beyond what we do in descriptions and inside of measures and other objects. The entire model. A single "page" that perhaps supported markdown but if it doesn't that's fine too, where we can just type what we want to and it is stored in the model. I don't know of there is a place to add an attribute to the model itself in the JSON/TMDL that would be safely ignored by the service. Perhaps similar to the way you added folder groupings.