Fix documentation about MySQL or MariaDB running as root

From the README: "Unfortunately, running MySQL as root (thus by default on docker) is not possible. MySQL (and MariaDB as well) will not allow it." The practice you're suggesting is the most secure and most common. However, the statement above is not correct. While it's just a detail, I suggest that you either delete or fix this claim. MySQL and MariaDB refuse to run as root **by default**, but we can force them by setting `user=root` in the configuration file.