[Spinal][servo] Create README for servo interface

aerial_robot_nerve/spinal/mcu_project/lib/Jsk_Lib/servo/README.md

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+# Spinal: Direct servo control
+## What is this
+This is a universal interface for servo motor drivers.
+## How to use
+### Dynamixel
+#### Setting
+Please follow these steps **in order**.
+##### 1. Connect Spinal and a power source to servo motors.
+##### 2. Reboot Spinal to find and initialize servo motors.
+- Just after this reboot, you can see the LEDs of the servos blink once.
+##### 3. Set basic parameters
+- TTL - RS485 mixed flag
+  - Flag on existence of mixed TTL and RS485 communication within the entire system.
+  - ``rosservice call /direct_servo_config "command: 6 data: [value (0 or 1)]" ``
+- Send data flag
+  - Flag on reading of servo data
+  - ``rosservice call /direct_servo_config "command: 4 data: [servo id, value (0 or 1)]" ``
+- External encoder flag
+  - Flag on the existence of external encoders.
+  - ``rosservice call /direct_servo_config "command: 7 data: [servo id, value (0 or 1)]" ``
+- Servo resolution ratio
+  - The ratio of joint resolution to servo resolution
+  - ``rosservice call /direct_servo_config "command: 8 data: [servo id, value (joint resolution), value (servo resolution)]" ``
+##### 4. Set Control parameters
+- Homing offset
+  - Homing offset of servo position.
+  - Range: -1,044,479 ~ 1,044,479
+  - ``rosservice call /direct_servo_config "command: 1 data: [servo id, value(offset)]" ``
+- PID gains
+  - PID gains for positional control
+  - Range: 0 ~ 16,383
+  - ``rosservice call /direct_servo_config "command: 2 data: [servo id, value(p gain), value(d gain), value(i gain)]" ``
+- Profile velocity
+  - Maximum velocity of servo
+  - Range: 0 ~ 32767 (0 represents infinity)
+  - ``rosservice call /direct_servo_config "command: 3 data: [servo id, value(velocity)]" ``
+- Current limit
+  - Limitation of servo current
+  - Range: 0 ~ 1,193
+  - ``rosservice call /direct_servo_config "command: 5 data: [servo id, value(current)]" ``