Add note about object key order in JSONField docs

[Lincoln]

Using JSONField with PostgreSQL, storing a dict in the field and then later retrieving it, will not preserve the key order,

because PostgreSQL jsonb type "does not preserve the order of object keys" (​https://www.postgresql.org/docs/current/datatype-json.html).

This behavior can be surprising, because python dict does preserve key order, and storing/retrieving dicts with json.dumps/json.loads will preserve key order.

So, I am proposing to add a note to the JSONField docs explaining that key order may not be preserved. This could go in either the current "PostgreSQL users" note, or a new note. This probably depends on if the other DB backends preserve key order. I haven't checked if any of the others do.