docs: update README

docs/using-the-postgresql-backend.md

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+# Using the PostgreSQL backend
+
+Now supports PostgreSQL backend.
+
+Because upstream Rclone image is based on `alpine 3.16`(and `linux/arm/v6` platform is based on `alpine 3.15`), it **only supports PostgreSQL 14 and previous versions**, see [Alpine 3.16 Packages](https://pkgs.alpinelinux.org/packages?name=postgresql*-client&branch=v3.16) and [Alpine 3.15 Packages](https://pkgs.alpinelinux.org/packages?name=postgresql*-client&branch=v3.15).
+
+<br>
+
+
+
+## Environment Variables
+
+#### DB_TYPE
+
+Set to `postgresql` switch to PostgreSQL database.
+
+Default: `sqlite`
+
+#### PG_HOST
+
+PostgreSQL host, **required**.
+
+#### PG_PORT
+
+PostgreSQL port.
+
+Default: `5432`
+
+#### PG_DBNAME
+
+PostgreSQL database name.
+
+Default: `vaultwarden`
+
+#### PG_USERNAME
+
+PostgreSQL username.
+
+Default: `vaultwarden`
+
+#### PG_PASSWORD
+
+PostgreSQL password, **required**.
+
+The login information will be saved in the `~/.pgpass` file.
+
+<br>
+
+
+
+## Backup
+
+Specify the above environment variables to switch to the PostgreSQL database.
+
+<br>
+
+
+
+## Restore
+
+When restoring, also specify the above environment variables to switch to the PostgreSQL database.
+
+1. Ensure that the database is accessible.
+
+Perhaps you will use the `docker-compose up -d [services name]` command to start the database separately.
+
+2. Verify that the `PG_HOST` you are using is accessible to.
+
+If your database is running in docker-compose, you need to find the corresponding network name via `docker network ls`  and add `--network=[name]` to the restore command to specify the network name.
+
+3. Restore and restart the container.