Adds documentation how permissions are granted when sharing boards, based on what I have found here: https://github.com/nextcloud/deck/blob/ea4655b24ed3b3ade5b00c126715d4e0f09ad5ae/lib/Service/PermissionService.php#L169 Signed-off-by: Cyrille Bollu <StCyr@users.noreply.github.com>