Improve POSTGRES_DB description (fixes #149)

Author: prodrigestivill

README.md

@@ -80,7 +80,7 @@ Most variables are the same as in the [official postgres image](https://hub.dock
 | BACKUP_KEEP_MONTHS | Number of monthly backups to keep before removal. Defaults to `6`. |
 | BACKUP_KEEP_MINS | Number of minutes for `last` folder backups to keep before removal. Defaults to `1440`. |
 | HEALTHCHECK_PORT | Port listening for cron-schedule health check. Defaults to `8080`. |
-| POSTGRES_DB | Comma or space separated list of postgres databases to backup. Required. |
+| POSTGRES_DB | Comma or space separated list of postgres databases to backup. If POSTGRES_CLUSTER is set this refers to the database to connect to for dumping global objects and discovering what other databases should be dumped (typically is either `postgres` or `template1`). Required. |
 | POSTGRES_DB_FILE | Alternative to POSTGRES_DB, but with one database per line, for usage with docker secrets. |
 | POSTGRES_EXTRA_OPTS | Additional [options](https://www.postgresql.org/docs/12/app-pgdump.html#PG-DUMP-OPTIONS) for `pg_dump` (or `pg_dumpall` [options](https://www.postgresql.org/docs/12/app-pg-dumpall.html#id-1.9.4.13.6) if POSTGRES_CLUSTER is set). Defaults to `-Z1`. |
 | POSTGRES_CLUSTER | Set to `TRUE` in order to use `pg_dumpall` instead. Also set POSTGRES_EXTRA_OPTS to any value or empty since the default value is not compatible with `pg_dumpall`. |