Document new commondata format

Created by: Zaharid

The proposal for the new commondata format lives in the heads of some of us, and various partial implementations across pull requests, some of which are different. Ideally we would have some RFC style document discussing both high level ideas such as motivation (do we still want it to be "for experimentalists"?) and complete implementation details (i.e. something with as much information as the validobj stubs).

I believe this would make it easier to discuss changes and to actually port to the new format.