Document parameter organization and naming conventions

See for example https://github.com/UBICenter/openfisca-us/pull/41#issuecomment-903836557

For example, one specific question: Should parameters include the attribute by which it's segmented? e.g. the standard deduction amount is currently named `filer` as that's what it's specified by. If not (e.g. if we called it `amount`) is there another way to hint the breakdown? We would ideally hint both the quantity type (e.g. amount vs thresholds) and the breakdown.

It'd probably be good to spec out the tree we're planning to fill in at the first ~3 levels of depth.