Update the SIL README file

libraries/Desktop/README

@@ -2,17 +2,32 @@ This provides some support files for building APM on normal desktop
 systems. This makes it possible to use debugging tools (such as gdb
 and valgrind) on the APM code
 
-To build it do this:
+The code can then run on the PC instead of on the Arduino board and
+simulate the behaviour of the real system by integrating it with
+X-Plane of FlightGear to build a Software-In-the-Loop (SIL) simulator.
 
-  cd ArduPlane
+It will use TCP sockets to communicate between the several software
-  make -f ../libraries/Desktop/Makefile.desktop hil
+components (ArduPilot, GCS and Flight simulator). All the ArduPilot
+serial ports that get initialised map to separate TCP ports, which
+means you can separately test the telemetry port and the main serial
+port. It also makes using a debugger easier, as the debugger can use
+stdin/stdout.
 
-currently only 'hil' builds work.
+So the new usage is:
 
-It currently runs with the first serial port mapped to
+ 1) build with "make -f ../libraries/Desktop/Makefile.desktop hil"
-stdin/stdout. To test it, you can use mavproxy like this:
 
- mavproxy.py --master=/tmp/ArduPlane/ArduPlane.elf
+ 2) start in a terminal like this: /tmp/ArduPlane.build/ArduPlane.elf
+    it will say something like this:
 
-that will run ArduPlane as a child process, and will give you the
+      Serial port 0 on TCP port 5760
-ability to control ArduPlane over MAVLink.
+      Waiting for connection ....
+
+ 3) start a GCS, pointing it at localhost:5760. For the current
+    mavproxy, you would use:
+
+       mavproxy.py --master=tcp:localhost:5760
+
+    MichaelO has also added support in the GCS mission planner for TCP.
+    You will see a TCP option in the drop down for the serial port, then
+    choose port 5760.