Documentation is a work in progress as I understand it. You can go by the examples contained in the Modules folder though.
If you start out fresh, make sure to have one set of files for every object (unit, building, leader, etc.) you introduce, rather than having all unit art in one file, all texts in another and so on as regular civ (any many mods) do.
Also make sure to not include any definitions you are not changing as these will be inherited from regular CIV (so you won't have to incorporate changes if the units get changed in the next patch).
Finally for those things you change (as opposed to add), only include a partial definition, i.e. only those xml tags you changed and enough xml tags to identify the object you changed.
I guess this is rather abstract right now, but when looking at the examples it should get clearer.