Added documentation for MySQL environment variables

Adds documentation for the MySQL environment variables that are checked for by the helper script run as the `ENTRYPOINT`. Fixes issue in [docker-library/mysql/issues/17](https://github.com/docker-library/mysql/issues/17).
"for the database" seems a bit vague, since we haven't mentioned `MYSQL_DATABASE` yet and there's no other "database" in this context; maybe something more like the following?

    for the database specified by the optional `MYSQL_DATABASE` variable