improved documentation within sample configuration file

sqlite_backup.conf-sample

@@ -10,6 +10,11 @@ BU_FROOT=preferred-basename
 DB_DIR=/path/to/database/directory
 DB_FILE=your-db.sqlite3
 #
+# docker compose settings
+DC_DIR=/path/to/directory/containing/your/docker-compose.yml 
+# this is the name at the top of your docker-compose.yml stanza describing the container running SQLite
+DC_CONT=name_of_your_container
+#
 # Reporting
 #
 # email address to send reports to, and subject