Add README

Signed-off-by: Guenter Roeck <linux@roeck-us.net>

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+Note
+====
+
+This driver is experimental and has only been tested on NCT6775F.
+
+This driver supercedes the NCT6775F and NCT6776F support in the W83627EHF
+driver.
+
+Kernel driver NCT6775
+=======================
+
+Supported chips:
+  * Nuvoton NCT6775F/W83667HG-I
+    Prefix: 'nct6775'
+    Addresses scanned: ISA address retrieved from Super I/O registers
+    Datasheet: Available from Nuvoton upon request
+  * Nuvoton NCT6776F
+    Prefix: 'nct6776'
+    Addresses scanned: ISA address retrieved from Super I/O registers
+    Datasheet: Available from Nuvoton upon request
+  * Nuvoton NCT6779D
+    Prefix: 'nct6776'
+    Addresses scanned: ISA address retrieved from Super I/O registers
+    Datasheet: Available from Nuvoton upon request
+
+Authors:
+        Guenter Roeck <linux@roeck-us.net>
+
+Description
+-----------
+
+This driver implements support for the Nuvoton NCT6775F, NCT6776F, and NCT6779D
+super I/O chips. We will refer to them collectively as Nuvoton chips.
+
+The chips implement 2 to 4 temperature sensors (9 for NCT6775F and NCT6776F),
+2 to 5 fan rotation speed sensors, 8 to 15 analog voltage sensors, one VID,
+alarms with beep warnings (control unimplemented), and some automatic fan
+regulation strategies (plus manual fan control mode).
+
+The temperature sensor sources on all chips are configurable. temp4 and higher
+attributes are only reported if its temperature source differs from the
+temperature sources of the already reported temperature sensors.
+The configured source for each of the temperature sensors is provided
+in tempX_label.
+
+Temperatures are measured in degrees Celsius and measurement resolution is 1
+degC for temp1 and and 0.5 degC for temp2 and temp3. For temp4 and higher,
+resolution is 1 degC for W83667HG-B and 0.5 degC for NCT6775F and NCT6776F.
+An alarm is triggered when the temperature gets higher than high limit;
+it stays on until the temperature falls below the hysteresis value.
+Alarms are only supported for temp1, temp2, and temp3.
+
+Fan rotation speeds are reported in RPM (rotations per minute). An alarm is
+triggered if the rotation speed has dropped below a programmable limit. Fan
+readings can be divided by a programmable divider (1, 2, 4, 8, 16, 32, 64 or
+128) to give the readings more range or accuracy. The driver sets the most
+suitable fan divisor itself. Some fans might not be present because they
+share pins with other functions.
+
+Voltage sensors (also known as IN sensors) report their values in millivolts.
+An alarm is triggered if the voltage has crossed a programmable minimum
+or maximum limit.
+
+The driver supports automatic fan control mode known as Thermal Cruise.
+In this mode, the chip attempts to keep the measured temperature in a
+predefined temperature range. If the temperature goes out of range, fan
+is driven slower/faster to reach the predefined range again.
+
+The mode works for fan1-fan4. Mapping of temperatures to pwm outputs is as
+follows:
+
+temp1 -> pwm1
+temp2 -> pwm2
+temp3 -> pwm3
+
+/sys files
+----------
+
+name - this is a standard hwmon device entry, it contains the name of
+       the device (see the prefix in the list of supported devices at
+       the top of this file)
+
+pwm[1-5] - this file stores PWM duty cycle or DC value (fan speed) in range:
+          0 (stop) to 255 (full)
+
+pwm[1-5]_enable - this file controls mode of fan/temperature control:
+       * 1 Manual mode, write to pwm file any value 0-255 (full speed)
+       * 2 "Thermal Cruise" mode
+       * 3 "Fan Speed Cruise" mode
+       * 4 "Smart Fan III" mode
+       * 5 "Smart Fan IV" mode
+
+       SmartFan III mode is not supported on NCT6776F and NCT6779D.
+
+pwm[1-5]_mode - controls if output is PWM or DC level
+        * 0 DC output (0 - 12v)
+        * 1 PWM output
+
+Thermal Cruise mode
+-------------------
+
+If the temperature is in the range defined by:
+
+pwm[1-4]_target    - set target temperature, unit millidegree Celsius
+                    (range 0 - 127000)
+pwm[1-4]_tolerance - tolerance, unit millidegree Celsius (range 0 - 15000)
+
+There are no changes to fan speed. Once the temperature leaves the interval,
+fan speed increases (temp is higher) or decreases if lower than desired.
+There are defined steps and times, but not exported by the driver yet.
+
+pwm[1-4]_min_output - minimum fan speed (range 1 - 255), when the temperature
+                      is below defined range.
+pwm[1-4]_stop_time  - how many milliseconds [ms] must elapse to switch
+                      corresponding fan off. (when the temperature was below
+                      defined range).
+
+Note: last two functions are influenced by other control bits, not yet exported
+      by the driver, so a change might not have any effect.
+
+Implementation Details
+----------------------
+
+TBD