Update documentation for installation of avro-kafka data adapters

The documentation here:
https://mariadb.com/kb/en/library/columnstore-streaming-data-adapters/
and any installation documentation needs to reflect apt-get repo addition
sudo add-apt-repository ppa:opencontrail/ppa

otherwise the apt-get install fails
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
mariadb-columnstore-kafka-avro-adapters : Depends: librdkafka++1 but it is not installable
E: Unable to correct problems, you have held broken packages.