hwmon: (tmp421) Add documentation

Add documentation for the tmp421 driver.
Signed-off-by: NAndre Prendel <andre.prendel@gmx.de>
Acked-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NJean Delvare <khali@linux-fr.org>

Documentation/hwmon/tmp421

+Kernel driver tmp421
+====================
+
+Supported chips:
+  * Texas Instruments TMP421
+    Prefix: 'tmp421'
+    Addresses scanned: I2C 0x2a, 0x4c, 0x4d, 0x4e and 0x4f
+    Datasheet: http://focus.ti.com/docs/prod/folders/print/tmp421.html
+  * Texas Instruments TMP422
+    Prefix: 'tmp422'
+    Addresses scanned: I2C 0x2a, 0x4c, 0x4d, 0x4e and 0x4f
+    Datasheet: http://focus.ti.com/docs/prod/folders/print/tmp421.html
+  * Texas Instruments TMP423
+    Prefix: 'tmp423'
+    Addresses scanned: I2C 0x2a, 0x4c, 0x4d, 0x4e and 0x4f
+    Datasheet: http://focus.ti.com/docs/prod/folders/print/tmp421.html
+
+Authors:
+	Andre Prendel <andre.prendel@gmx.de>
+
+Description
+-----------
+
+This driver implements support for Texas Instruments TMP421, TMP422
+and TMP423 temperature sensor chips. These chips implement one local
+and up to one (TMP421), up to two (TMP422) or up to three (TMP423)
+remote sensors. Temperature is measured in degrees Celsius. The chips
+are wired over I2C/SMBus and specified over a temperature range of -40
+to +125 degrees Celsius. Resolution for both the local and remote
+channels is 0.0625 degree C.
+
+The chips support only temperature measurement. The driver exports
+the temperature values via the following sysfs files:
+
+temp[1-4]_input
+temp[2-4]_fault