update readme

README.md

@@ -81,6 +81,15 @@ Retention/compression:
 - `observations_ws90` has a 90‑day retention policy and compression after 7 days.
 - `observations_baro` has a 90‑day retention policy and compression after 7 days.
 
+### Existing databases
+If you’re on an existing database, you’ll need to apply the new table definition once (the init SQL only runs on a fresh DB). Example:
+
+```sh
+docker compose exec -T timescaledb psql -U postgres -d micrometeo -f /docker-entrypoint-initdb.d/001_schema.sql
+```
+
+Or copy just the `observations_baro` section into a manual `psql -c`.
+
 ## Publish a test WS90 payload
 ```sh
 mosquitto_pub -h localhost -t ecowitt/ws90 -m '{"model":"Fineoffset-WS90","id":70618,"battery_ok":1,"battery_mV":3180,"temperature_C":24.2,"humidity":60,"wind_dir_deg":129,"wind_avg_m_s":0,"wind_max_m_s":0,"uvi":0,"light_lux":0,"flags":130,"rain_mm":0,"rain_start":0,"supercap_V":0.5,"firmware":160,"data":"3fff000000------0000ff7ff70000","mic":"CRC","protocol":"Fine Offset Electronics WS90 weather station","rssi":-44,"duration":32996}'