Please add ZUGFeRD invoice in Readme

[fank]

I just noticed you added an example with 2761ea3 looked at the readme before and didn't noticed it.

[fank]

Also the image in the readme of it is quite hard to read/identify in github darkmode:

[pgundlach]

I am not sure I understand this issue. I see that the image is crappy (transparent background), but which Readme to add to?

[fank]

the main one https://github.com/speedata/examples/blob/master/Readme.md

[fank]

Let me rephrase it, I was looking through the readme for any sales or purchase document example, didn't found any first so I thought "sad it doesn't support".
Because those documents require the page break logic, which a lot of PDF converter do not support.