MariaDB uses socket authentication in Debian unstable. That is an important feature that increases security and makes administration easier. In the majority of cases you don't need or want to make a root user, only user accounts that have limited and intended access to the database.
I don't know what README.Debian you refer to. The mariadb-10.0 does not have any such file. I will close this bug report as invalid, because what you report about is not a bug, maybe just a misunderstanding. Please feel free to contribute with documentation, there could maybe be a README.Debian file in the package that explains something about how authentication works.
