Add README.md for brief information of sensord

To make sensord more informative, the README.md is added to provide
brief information to the sensord users.
Since the fusion sensor information is required, it is added first.

Change-Id: I51678d9a1ae125ad8df3b62e99d01e7ca70c8799
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>

diff --git a/README.md b/README.md
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+# sensord (work in progress)
+sensord is a daemon that manages sensors and provides values from sensors to clients.
+
+## Index
+* [sensors](#sensors)
+  * [Physical sensor](#physical_sensor)
+  * [Fusion sensor](#fusion_sensor)
+    * [Required sensors per fusion sensor](#required_sensors_per_fusion_sensor)
+  * [Virtual sensor](#virtual_sensor)
+
+## sensors
+There are various types of sensors in sensord.
+
+<div id='physical_sensor'/>
+
+### Physical sensor
+...
+
+<div id='fusion_sensor'/>
+
+### Fusion sensor
+Fusion sensor is a sensor type that combines values from multiple sensors and
+then generate new sensor value.\
+For example, auto_rotation sensor reads accelerometer values and reproduces them
+into rotation values(0, 90, 180, 270 degrees).\
+Since fusion sensors uses other sensors, if some sensors are not available, then
+fusion sensors that uses these sensors become unavailable too.
+
+<div id='required_sensors_per_fusion_sensor'/>
+
+#### Required sensors per fusion sensor
+* gravity_comp_sensor (http://tizen.org/sensor/general/gravity/tizen_complementary)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+  * gyroscope (http://tizen.org/sensor/general/gyroscope)
+* gravity_lowpass_sensor (http://tizen.org/sensor/general/gravity/tizen_lowpass)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+* linear_accel_sensor (http://tizen.org/sensor/general/linear_acceleration)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+  * gravity (http://tizen.org/sensor/general/gravity)
+* auto_rotation_sensor (http://tizen.org/sensor/general/auto_rotation/tizen_default)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+* pick_up_sensor (http://tizen.org/sensor/general/gesture_pick_up/tizen_default)
+  * gesture_movement (http://tizen.org/sensor/general/gesture_movement/SENSOR_GESTURE_MOVEMENT)
+* face_down_sensor (http://tizen.org/sensor/general/gesture_face_down/tizen_default)
+  * gravity (http://tizen.org/sensor/general/gravity)
+* orientation_sensor (http://tizen.org/sensor/general/orientation/tizen_default)
+  * rotation_vector (http://tizen.org/sensor/general/rotation_vector)
+* gyro_orientation_sensor (http://tizen.org/sensor/general/gyroscope_orientation/tizen_default)
+  * gyroscope_rotation_vector (http://tizen.org/sensor/general/gyroscope_rotation_vector)
+* magnetic_orientation_sensor (http://tizen.org/sensor/general/geomagnetic_orientation/tizen_default)
+  * geometric_rotation_vector (http://tizen.org/sensor/general/geomagnetic_rotation_vector)
+* pedometer_sensor (http://samsung.com/sensor/healthinfo/pedometer/samsung_pedometer)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+* gyro_rv_sensor (http://tizen.org/sensor/general/gyroscope_rotation_vector/tizen_default)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+  * gyroscope (http://tizen.org/sensor/general/gyroscope)
+* rv_sensor (http://tizen.org/sensor/general/rotation_vector/tizen_default)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+  * gyroscope (http://tizen.org/sensor/general/gyroscope)
+  * magnetic (http://tizen.org/sensor/general/magnetic)
+* magnetic_rv_sensor (http://tizen.org/sensor/general/geomagnetic_rotation_vector/tizen_default)
+  * accelerometer (http://tizen.org/sensor/general/accelerometer)
+  * magnetic (http://tizen.org/sensor/general/magnetic)
+
+<div id='virtual_sensor'/>
+
+### Virtual sensor