In many cases the documentation for MySQL can be used. New features of MariaDB are mentioned on the MariaDB versus MySQL page and in greater detail here on AskMonty.org in the MariaDB category.
If it's a question about a feature that is not properly documented or something that is not working as expected, go to the corresponding knowledge base page and ask your question there.
You can report and check on bugs which are unique to MariaDB in Launchpad. MySQL bugs that also affect MariaDB should be reported to MySQL.
You can also subscribe to the mailing lists and or join the IRC channel to communicate with other MariaDB users.
Monty Program Ab offers commercial support for MariaDB and all major MySQL versions (starting from MySQL 3.23) through a worldwide network of providers.