docs: update README

README.md

@@ -385,6 +385,16 @@ Default: `/bitwarden/data`
 
 > **You don't need to change these environment variables unless you know what you're doing.**
 
+#### BACKUP_FILE_DATE
+
+Edit this environment variable only if you explicitly want to change the time prefix of the backup file (e.g. 20220101). **Incorrect configuration may result in the backup file being overwritten by mistake.**
+
+Example of possible scenario: When performing monthly or weekly backups, you want only month or week information to appear on the backup file name.
+
+See [`BACKUP_FILE_DATE_SUFFIX`](#backup_file_date_suffix) for details.
+
+Default: `%Y%m%d`
+
 #### DATA_DB
 
 Set the sqlite database file path.