Documentation should describe nma_pk_* transfer-only columns on Ocotillo core tables

[kbighorse]

Ocotillo core tables (Thing, Location, Sample, Observation, Contact, Sensor) have nma_pk_* columns that exist only for transfer/backfill from NM_Aquifer. They are not part of the public API (response schemas no longer expose them).

Requested action: Add a short section to the repo (e.g. in transfers/README.md or a new db/README.md) that:

• States that these columns are used only by transfers and backfill.
• Notes that some values are not preserved elsewhere (e.g. Observation.nma_pk_waterlevels, Contact.nma_pk_owners, Sensor.nma_pk_equipment, Thing.nma_pk_wellscreens).
• Warns: do not drop these columns without a migration that preserves or explicitly retires that data.

This prevents future "cleanup" from dropping columns and losing data, and makes the split between "new schema" and "transfer" explicit.