Document MySQL port availability before startup

[93jun]

This adds a short Requirements note that the configured MySQL/MariaDB port must
be available before the role starts the service.

The role already documents mysql_enabled_on_startup and the default
mysql_port: "3306". If another MySQL/MariaDB instance or local service is
already bound to the configured port, startup can fail or later database module
operations may not connect to the expected service.

This change documents the operational precondition directly in the README:

• when mysql_enabled_on_startup is true, ensure mysql_port is available
• stop or reconfigure any existing MySQL/MariaDB instance using the same port
• set mysql_port to an available port before startup if needed

Validation:

git diff --check