staging:iio:Documentation Simple dummy driver to explain the basics

The documenation explaining how to go about writing a driver is lagging
horribly, so here is another approach; an actual driver with
lots of explanatory comments.

Note it is currently minimal in that there are no events and no
buffer.  With care they can probably be added in additional files
without messing up the clarity of what we have here.

V2: Addressed some of Manuel Stahl's feedback.

Fixed up kernel doc.
Added more general description.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>