Add README's to hdfs dirs

[sverhoeven]

When automated Docker hub has been configured the README.md next to the Dockerfile is used as the full description. For example xenon-ssh/README.md is shown at https://hub.docker.com/r/nlesc/xenon-ssh/

The descriptions of https://hub.docker.com/r/nlesc/xenon-hdfs/ and https://hub.docker.com/r/nlesc/xenon-hdfs-kerberos/ are empty.

Please add README to the hdfs dirs similar to the other README's.