Schema style guide: add note on denormalization

[duncandewhurst]

From https://crm.open-contracting.org/issues/7721#note-40:

Having two possible data elements for the same fact also leads to potential inconsistent values. We generally only de-normalize or duplicate data in cases where the information is external (for example, supplier names could in theory be left out and deferred to a corporate registry, as long as an identifier is published).

It would be good to document this in the style guide.