expanded README

README.md

@@ -93,3 +93,7 @@ Parsing error: Unexpected token:
 React.FC<P & ButtonWithClickOnceProps>
            ^
 ```
+
+## E-invoice xml
+
+The e-invoices generated by confac should comply with the Peppol BIS Billing 3.0 standard defined by the EU. To doublecheck if the created xml does follow this protocol, you can use this [tool](https://ecosio.com/en/peppol-and-xml-document-validator/). Choose 'OpenPeppol UBL Invoice (2023.05) (aka BIS Billing 3.0.15)' as the ruleset. This is the most current ruleset at the time of writing. Release notes and the general xml structure can be consulted [here]( https://docs.peppol.eu/poacc/billing/3.0)