Update README.adoc

Document concept.

pio/pio_servo/README.adoc

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 Drive a servo motor by using two PIO statemachines for pulse output.
 
+== Concept
+
+Servo motors are controlled by a pulse with modulated signal.
+Please see Wikipedia for details: https://en.wikipedia.org/wiki/Servo_control
+
+The angle is encoded by the pulse length. The positive pulse length from 1ms to 2ms describes the movement from minimum to maximum.
+This pulse has to be repeated every 20ms.
+
+This pulseform is generated by two different PIO statemachines.
+
+Statemachine 1, running the servo_trigger program generates the 20ms carrier signal.
+It counts down for 20ms, then clears the IRQ, allowing the 2nd statemachine to run.
+
+Statemachine 2, running servo_prog waits for the IRQ to be cleared.
+The cleared IRQ marks the beginning of a 20ms cycle, which means, that the pulse needs to be output.
+After this, the statemachine outputs a pulse of 1 to 2 milliseconds.
+The duration of this pulse can be input from the FIFO.
+If a value of 0 is pushed to the FIFO, the output pulse will have 1ms, which is the lowest movevement position of the servo.
+Pushing a value of 1000 to the FIFO outputs a 2ms pulse, which moves the servo to the maximum position.
+
+After the pulse has been output, Statemachine 2 waits for Statemachine 1 to clear the IRQ again, so the next pulse can be output.
+
+== PIO Use
+
 Each output needs a statemachine running servo_trigger and one running servo_prog.
 Due to the IRQ assignments, the statemachines need to be in ordered to have the servo_trigger program first, e.g.: