README: indicate change to ENV for configuration, refresh systemd unit file, introduce Gorm and db migration tool

README.md

@@ -34,6 +34,9 @@ https://rushlink.local` to the binary (e.g. in the `systemd` unit file).
 
 ## Sample `systemd` unit file
 
+As of 1fe9553cc9, `rushlink` expects its database and file store configuration
+in environment variables.
+
 ```
 [Install]
 WantedBy=nginx.service
@@ -42,7 +45,8 @@ WantedBy=nginx.service
 Type=simple
 User=rushlink
 Group=nogroup
-ExecStart=/var/lib/rushlink/rushlink -database /var/lib/rushlink/db -file-store /var/lib/rushlink/filestore -root_url https://rushlink.local
+Environment=RUSHLINK_DATABASE_DRIVER=sqlite RUSHLINK_DATABASE_PATH=/var/lib/rushlink/rushlink.sqlite3 RUSHLINK_FILE_STORE_PATH=/var/lib/rushlink/filestore/
+ExecStart=/var/lib/rushlink/rushlink -root_url https://rushlink.local
 ```
 
 ---
@@ -65,10 +69,16 @@ are the exceptions:
 
 ## Database
 
-We use [`go.etcd.io/bbolt`]. This file should be the *only* file
+Before 1fe9553cc9 we used [`go.etcd.io/bbolt`]. This file should be the *only* file
 apart from our monolithic binary. All settings and keys should go in here.
 Any read-only data resides in the binary file (possibly compressed).
 
+Now, we use Gorm and support SQLite and PostgreSQL as backends.
+
+We provide a migration binary in `cmd/rushlink-migrate-db/`. In environment
+variables, the destination database is configured, and the binary itself
+expects a few flags. Refer to the source for the exact flags.
+
 [`go.etcd.io/bbolt`]: `go.etcd.io/bbolt`
 
 ## Namespacing