README: fix variables/playbook

README.md

@@ -16,6 +16,7 @@ Variable | Context | Description
 `db_repl_user` | (defaults) | Repliation user name
 `db_repl_password` | (defaults) | Replication password
 `db_debug` | (undefined) | Print some debug information
+`hostname_replication_primary` | (REQUIRED/undefined) hostname of primary
 
 Dependencies
 ------------
@@ -29,6 +30,7 @@ Example Playbook
       roles:
         - role: mariadb-replication
           vars:
+            hostname_replication_primary: groups.db-servers[0]
             db_debug: true
 
 License