Comment on performance of objecttable and arraytable

Simply doing `JSON3.write(Tables.columntable(::DataFrame))` or `JSON3.write(Tables.rowtable(::DataFrame))` outperforms the functions of JSONTables for me.

But only tested with non-custom eltypes (Numbers, Strings, etc.).

Maybe this could be worth a note in the docs somewhere.

But thank you very much for your packages!!

regards