Documentation unclear on product registration.

[danielr1996]

The documentation states that a registration is necessary, but not where to do so and where to specify the productId.

The PinTanClient has an optional field productId where I could put the productId, but since ~2018 FinTS made the productId mandatory so it would be nice to update the PinTanClient to make the productId also mandatory.

Also a link in the documentation to https://www.hbci-zka.de/register/prod_register.htm, where you can get a productId, would be nice.

I found the information in libfintx/libfintx#53 where the author of libfintx also kindly provided his registration number to use.