SITL: Added Readme for Airsim setup & usage

libraries/SITL/SITL_AirSim.md

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+## AirSim Setup
+
+This is a temporary page describing the development setup of AirSim and how to use Ardupilot SITL with it (currently under development)
+
+### Linux
+
+This AirSim page describes how to build Unreal Engine, AirSim
+<https://microsoft.github.io/AirSim/docs/build_linux/>
+
+Setting up the Blocks Environment - <https://microsoft.github.io/AirSim/docs/unreal_blocks/>
+
+Development Workflow in AirSim -  
+
+- Updating the repo - Normal Git workflow
+- Make any changes required
+- Run the `build.sh` script in Airsim which will also copy the changes to the plugin directory
+- Launch UnrealEngine Editor, choose Blocks environment and when prompted about missing .so files, press Yes to build it again.
+
+- Troubleshooting - <https://microsoft.github.io/AirSim/docs/build_linux/>
+
+### Windows
+
+Build AirSim on Windows - <https://microsoft.github.io/AirSim/docs/build_windows/>
+
+Setup Blocks Environment - <https://microsoft.github.io/AirSim/docs/unreal_blocks/>
+
+Development Workflow - <https://microsoft.github.io/AirSim/docs/dev_workflow/>
+
+### Using ArduCopter vehicle
+
+Use the following branch for ArduCopter vehicle - <https://github.com/rajat2004/AirSim/tree/pr-arducopter>  
+
+Note: Has been tested on a single Linux Machine running Ubuntu 16.04. The branch should compile with Windows as well but hasn't been tested yet.
+
+Before launching for the first time, go to `Edit->Editor Preferences`, in the 'Search' box type `CPU` and ensure that the `Use Less CPU when in Background` is unchecked.
+
+Press Play after Editor loads.
+
+Initially, the editor will hang after pressing the Play button if the Ardupilot SITL hasn't been started. Run `sim_vehicle.py` and it should go back to normal
+
+See <https://github.com/Microsoft/AirSim/blob/master/docs/settings.md> for info about settings
+
+Current `settings.json` file for launching Arducopter
+
+```
+{
+  "SettingsVersion": 1.2,
+  "LocalHostIp": "127.0.0.1",
+  "LogMessagesVisible": true,
+  "SimMode": "Multirotor",
+  "OriginGeopoint": {
+    "Latitude": -35.363261,
+    "Longitude": 149.165230,
+    "Altitude": 583
+  },
+  "Vehicles": {
+      "Copter": {
+          "VehicleType": "ArduCopter",
+          "UseSerial": false,
+          "AllowAPIAlways": false,
+          "UdpIp": "127.0.0.1",
+          "UdpPort": 9003,
+          "SitlPort": 9002 
+        }
+    }
+}
+```
+
+Note: Many of the fields have the default values as above only but just specifying them. If you want to select a specific local network adapter so you can reach certain remote machines (e.g. wifi versus ethernet), then you will want to change the `LocalHostIp` accordingly.
+
+First launch AirSim, after that launch the ArduPilot SITL using 
+
+```
+sim_vehicle.py -v ArduCopter -f airsim-copter --add-param-file=libraries/SITL/examples/Airsim/quadX.parm --console --map
+```
+
+For closing, first stop the AirSim simulation by pressing the Stop button, then close Ardupilot.  
+If Ardupilot is closed first, then UE hangs and you'll need to force close it.  
+You can restart by just pressing the Play button and then start the Ardupilot side, no need to close the Editor completely and then start it again
+
+
+#### Run on different machines
+Change `UdpIp` to the IP address of the machine running Ardupilot, use `--sim-address` to specify Airsim's IP address
+
+
+Using different ports -  
+`UdpPort` denotes the port no. which Ardupilot receives the sensor data on (i.e. the port that Airsim sends the data to)  
+`SitlPort` assigns the motor control port on which Airsim receives the rotor control message
+
+- `--sim-port-in` should be equal to sensor port i.e. `UdpPort`  
+- `--sim-port-out` should be equal to motor control port i.e. `SitlPort`