SITL: examples: add JSON readme

libraries/SITL/examples/JSON/readme.md

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+The JSON SITL backend allows software to easily interface with ArduPilot using a standard JSON interface.
+
+To launch the JSON backend run SITL with ```-f json:127.0.0.1``` where 127.0.0.1 is replaced with the IP the physics backend is running at.
+
+Connection to SITL is made via a UDP link, port 9002 should be used to receive inputs from ArduPilot and port 9003 to return attitude information to ArduPilot.
+
+SITL output
+Data is output from SITL in a binary format:
+```
+    uint32_t frame_count
+    float Speedup
+    uint16_t pwm[16] 
+```
+The frame_count will increment for each output frame sent by ArduPilot, this count can be used to detect lost or duplicate frames. This count will be reset when SITL is re-started allowing the physics backend to reset the vehicle. If not input data is received after 10 seconds ArduPilot will re-send the output frame without incrementing the counter. This allows the physics model to be restarted and re-connect. Note that this may fill up the input buffer of the physics backend after some time. Speed up is the desired speed up as set with the SIM_SPEEDUP param, this is a target only. PWM is a array of 16 servo values in micro seconds, typically in the 1000 to 2000 range as set by the servo output functions.
+
+SITL input
+Data is received from the physics backend in a plain text JSON format. The data must contain the following fields:
+```
+    timestamp (us) physics time
+    imu:
+        gyro(roll, pitch, yaw) (radians/sec) body frame
+        accel_body(north, east, down) (m/s^2) body frame
+
+    position(north, east, down) (m) earth frame
+    attitude(roll, pitch yaw) (radians)
+    velocity(north, east,down) (m/s) earth frame
+```
+This is a example input frame, it should be preceded by and terminated with a carriage return ("\n") :
+```
+{"timestamp":2500,"imu":{"gyro":[0,0,0],"accel_body":[0,0,0]},"position":[0,0,0],"attitude":[0,0,0],"velocity":[0,0,0]}
+```
+The order of fields is not important, this is minimum required fields. In the future a support for a number of additional optional felids will be added to allow readings to be provided for sensors such as airspeed and lidar.