README: Added description for mysql/backup_database

README.md

@@ -27,6 +27,7 @@ Following roles have been defined to make creating a server configuration easy:
   - **system_user** creates a system user
 - **mysql** defines roles for handling mysql databases and users, uses *MariaDB*
   - **application** installs the main application with automatic backup
+  - **backup_database** configures auto backup for a given mysql database
   - **database** configures a database for an external application with its own user
 - **nginx** defines roles to set up virtual servers, certificates will be requested by default
   - **application** installs and configures the main requirements