Add information about duress PIN

[xmready]

Please consider adding information about the length of the duress PIN when/if auto-confirm is enabled. This information could be added to the already existing Duress section on the Features page, and possibly also to a new section on the Usage page.

Auto-confirm will prevent the input of a duress PIN if the duress PIN is longer than the normal PIN. This may seem obvious to some, but it was not obvious to me. If I hadn't tested this myself, my duress PIN would have failed in a real world scenario when I would have needed it.

Thanks