Update README: "Third party validation configurations"

[fbuettner-hb]

That section is outdated. There are many more validation configuration than the two currently listed in the README file (and one of the two, "XGewerbeanzeige", is outdated as that specification has been renamed and the link is broken). It is not feasible that list up to date in this project.

Please update that section.

Proposal:

There are several third party validation configurations available. For example for XRechnung - Source code: GitHub, Release: Releases.