w1_therm: adding sysfs-driver-w1_therm doc

Adding a sysfs-driver-w1_therm documentation file in
Documentation/ABI/testing. It describe the onlys sysfs entry of w1_therm
module, based on Documentation/w1/slaves/w1_therm.rst

Signed-off-by: Akira Shimahara <akira215corp@gmail.com>
Link: https://lore.kernel.org/r/20200511203631.410227-1-akira215corp@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>