added readme for hardware layer

hardware_layer/README.md

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+# Hardware Layer
+
+## Micro-controller
+
+__Hardware__: Arduino Mega 2560
+
+__Purpose__:  Subscribes to the PWM values published from the [motion_library_layer](https://github.com/AUV-IITK/auv2016/tree/master/motion_library_layer) and sends it to the Arduino to apply variable voltages on thrusters accordingly, also publishes data from the depth sensor
+
+* Subscribe data on four different topics for four different motions. And publish
+pressure sensor data on a topic.
+* When a PWM is subscribed on a topic, it’s callback function is called. It first
+apply calibration on the PWM to get the individual PWM of each thruster, then
+send it to the thrusters.
+* In case of turn motion, it uses side thursters if robot is moving sideward also, else
+it uses thursters at end.
+
+## Camera and Image Processing
+
+__Hardware__: Logitech Webcam
+
+__Purpose__: Takes the raw data from the two cameras (front and bottom) and publish them on two different topics for processing on task
+nodes. It is done for each frame at a certain rate.
+
+## Inertial Measurement Unit (IMU)
+
+__Hardware__: [Navstick Nanopilot Board](https://github.com/AUV-IITK/AUVWiki/wiki/Using-NavStick-NanoPilot-Board)
+
+__Purpose__: Publish the angle from the North axis.
+
+* Takes data from the navstik port.
+* Runs calculations to compute the angle from North axis.
+* Publishes the angle on a topic to motion library and task handlers.