Please add release tags

Hi,
I just realised by chance that ncl moved from SourceForge (latest version 2.1.18) to Github (specifying 2.1.21 in README.md).  I would love to update the Debian package to the most recent version but to do so it would really help if you would add release tags to enable us spotting if new code is ready for a release distribution.
Thanks for considering, Andreas.