[FEATURE] Add example of adding extra fields to User Settings

Resolves: #57090
Releases: 0.6.0
Change-Id: I6abe164a7ef3a8fd6a555162df5d6315f705b460
Reviewed-on: https://review.typo3.org/28544
Reviewed-by: Francois Suter
Tested-by: Francois Suter