The biggest news is that ZUGFeRD 2.1 now also supports the filename
factur-x.xml for the embedded XML.
Which means that you can already create ZF 2.1 with the current version
of Mustang with sth like setZUGFeRDVersion(2) or the command line tool,
first of all because the version indicator 2p0 did not change, and
secondly because you can select factur-x output (sth like setFacturX()).
I might release a new sample invoice and maybe a polished Mustangproject
with changed defaults (factur-x to be auto selected with the possibility
of a manual override to zugferd-invoice.xml) but in general you should
already be ready to go.