dell-smm-hwmon: Add documentation

dell-smm-hwmon: Add documentation

Part of the documentation is taken from the README of the userspace
utils (https://github.com/vitorafsr/i8kutils). The license is GPL-2+
and the author Massimo Dal Zotto is already credited as author of
the module. Therefore there should be no copyright problem.

I also added a paragraph with specific information on the experimental
support for automatic BIOS fan control.

Signed-off-by: Giovanni Mascellani <gio@debian.org>
Link: https://lore.kernel.org/r/20191122101519.1246458-2-gio@debian.org
[groeck: Fixed some of the documentation warnings]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>