Update howto-dm-db.Rmd

[andreranza]

In light of the latest features implemented in version 0.3.0 (#1106, #1123, #1169), we update the vignette "howto-dm-db.Rmd".

There is presented an example which relies on MariaDB, for which - previous to 0.3.0 -was not possible to import primary and foreign keys automatically.

For the moment, in order not to change the entire example in which pks and fks are added manually, we adapt the documentation as it would still not be possible to import them and highlight the fact that from 0.3.0 things have changed in this respect.

Maybe change example in the future?