Add more info about box

[marekrogala]

Jakub's comment on PR #130:

Both __init__.R and reexports are nice features that we will utilize in the future but for now, it seems to me that there is too much weight put here. As written above, I would recommend adding some additional descriptions and examples to basic box usage. It would be good to also wrap this part in an additional section (e.g. Additional box features) and by that, reduce header levels of init and reexport.