um: Document dtb command line option

Add documentation for the dtb command line option and the
ability to load/parse device trees.
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Reviewed-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

Documentation/virt/uml/user_mode_linux_howto_v2.rst

@@ -1189,6 +1189,26 @@ E.g. ``os_close_file()`` is just a wrapper around ``close()``
 which ensures that the userspace function close does not clash
 with similarly named function(s) in the kernel part.
 
+Using UML as a Test Platform
+============================
+
+UML is an excellent test platform for device driver development. As
+with most things UML, "some user assembly may be required". It is
+up to the user to build their emulation environment. UML at present
+provides only the kernel infrastructure.
+
+Part of this infrastructure is the ability to load and parse fdt
+device tree blobs as used in Arm or Open Firmware platforms. These
+are supplied as an optional extra argument to the kernel command
+line::
+
+    dtb=filename
+
+The device tree is loaded and parsed at boottime and is accessible by
+drivers which query it. At this moment in time this facility is
+intended solely for development purposes. UML's own devices do not
+query the device tree.
+
 Security Considerations
 -----------------------