Document Append-only

Test is here https://github.com/MeltanoLabs/target-postgres/blob/main/target_postgres/tests/test_target_postgres.py#L384 but we should give a more realistic use case for how this can be used with Meltano

See slack thread here https://meltano.slack.com/archives/C069CQNHDNF/p1712154286510799