Documentation: iio-trig-sysfs: Document add_trigger attribute

This patch adds the ABI documentation for the add_trigger
attribute, which is provided by the iio-trig-sysfs stand-alone driver.
Signed-off-by: NCristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-trigger-sysfs

@@ -18,3 +18,14 @@ Description:
 		trigger. In order to associate the trigger with an IIO device
 		one should write this name string to
 		/sys/bus/iio/devices/iio:deviceY/trigger/current_trigger.
+
+What:		/sys/bus/iio/devices/iio_sysfs_trigger/add_trigger
+KernelVersion:	2.6.39
+Contact:	linux-iio@vger.kernel.org
+Description:
+		This attribute is provided by the iio-trig-sysfs stand-alone
+		driver and it is used to activate the creation of a new trigger.
+		In order to achieve this, one should write a positive integer
+		into the associated file, which will serve as the id of the
+		trigger. If the trigger with the specified id is already present
+		in the system, an invalid argument message will be returned.