The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
Many modules packaged for the R Project for Statistical Computing contain data files with names as *.rda, *.Rda, *.rdata, *.Rdata, etc.
When such files exist, the FTP masters expect them to be explained in debian/README.source, which this package is missing.
Please add a README.source documenting the origins of these files.
Please refer to https://lists.debian.org/debian-devel/2013/09/msg00332.html for details.