Documentation: ABI: Add ABI file for legacy /proc/i8k interface

Add ABI file for informing remaining users of the
deprecation of the legacy /proc/i8k interface.
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20220109214248.61759-4-W_Armin@gmx.deSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/ABI/obsolete/procfs-i8k

+What:		/proc/i8k
+Date:		November 2001
+KernelVersion:	2.4.14
+Contact:	Pali Rohár <pali@kernel.org>
+Description:	Legacy interface for getting/setting sensor information like
+		fan speed, temperature, serial number, hotkey status etc
+		on Dell Laptops.
+		Since the driver is now using the standard hwmon sysfs interface,
+		the procfs interface is deprecated.
+Users:		https://github.com/vitorafsr/i8kutils

MAINTAINERS

@@ -5410,6 +5410,7 @@ F:	drivers/platform/x86/dell/dell-rbtn.*
 DELL LAPTOP SMM DRIVER
 M:	Pali Rohár <pali@kernel.org>
 S:	Maintained
+F:	Documentation/ABI/obsolete/procfs-i8k
 F:	drivers/hwmon/dell-smm-hwmon.c
 F:	include/uapi/linux/i8k.h