DocBook: Add initial documentation for IIO

This is intended to help developers faster find their way
inside the Industrial I/O core and reduce time spent on IIO
drivers development.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Acked-by: Crt Mori <cmo@melexis.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/DocBook/Makefile

@@ -15,7 +15,7 @@ DOCBOOKS := z8530book.xml device-drivers.xml \
 	    80211.xml debugobjects.xml sh.xml regulator.xml \
 	    alsa-driver-api.xml writing-an-alsa-driver.xml \
 	    tracepoint.xml drm.xml media_api.xml w1.xml \
-	    writing_musb_glue_layer.xml crypto-API.xml
+	    writing_musb_glue_layer.xml crypto-API.xml iio.xml
 
 include Documentation/DocBook/media/Makefile