Change db volume version, and add hints to upgrade it in the future

This isn't strictly necessary, this is just artifacts of the upgrade
process I did—and I figure may as well leave them here to help me in
the future if I need to do it again!

files/docker-compose.yml

@@ -5,7 +5,7 @@ services:
     image: postgres:16-alpine
     restart: always
     volumes:
-      - db-data:/var/lib/postgresql/data
+      - db-data-v16:/var/lib/postgresql/data
     environment:
       - POSTGRES_PASSWORD=postgres
 
@@ -35,7 +35,13 @@ services:
       - plausible-conf.env
 
 volumes:
-  db-data:
+  # NOTE: When it comes time to upgrade Postgres, you should:
+  #   1. Manually export the database with `pg_dumpall` (via `docker exec`).
+  #   2. Change the version number of this volume, to create a fresh data directory.
+  #   3. Upgrade the Postgres version, and don't let Plausible itself run yet (so it doesn't init an empty DB).
+  #   4. Import the database with `psql` (via `docker exec`).
+  #   5. (Optional) Remove the previous volume with `docker volume rm db-data-vXX`.
+  db-data-v16:
     driver: local
   event-data:
     driver: local