Update README with explaining of conditional enabling/disabling role

Author: fitz123

README.md

@@ -21,6 +21,7 @@ This role focuses on security configuration of MySQL. Therefore you can add this
 
 This hardening role installs the hardening but expects an existing installation of MySQL, MariaDB or Percona. Please ensure that the following variables are set accordingly:
 
+- `mysql_hardening_enabled: yes` role is enabled by default and can be disabled without removing it from a playbook. You can use conditional variable, for example: `mysql_hardening_enabled: "{{ true if mysql_enabled else false }}"`
 - `mysql_hardening_user: 'mysql'` The user that mysql runs as.
 - `mysql_datadir: '/var/lib/mysql'` The MySQL data directory
 - `mysql_hardening_hardening_conf: '/etc/mysql/conf.d/hardening.cnf'` The path to the configuration file where the hardening will be performed