Add GPIO doc (#16)

* Add GPIO doc

reference/gpio.md

@@ -0,0 +1,26 @@
+---
+layout: reference
+title: GPIO pins
+tags: 
+- gpio
+---
+
+## List of GPIO pins
+
+These GPIOs are accessible through their kernel interface in `/sys/class/gpio/gpioXXX`.
+For more information on how to use that interface, see Linux documentation:
+https://www.kernel.org/doc/Documentation/gpio/gpio.txt
+
+|Name|Kernel number|Direction|Polarity|Description|
+|----|------|----|----|-----|
+|VOL_UP|404|in|active low|volume up button|
+|VOL_DOWN|402|in|active low|volume down button|
+|HEAD_nRESET|385|out|active high|set to low to reset head board|
+|LS_nRESET|392|out|active high|set to low to reset low speed board|
+|CHEST_nRESET|394|out|active high|set to low to reset volume board|
+|WIFI_nRESET|401|out|active high|set to low to reset wifi chip|
+|AUDIO_nRESET|405|out|active high|set to low to reset audio chip|
+|HEAD_GPIO_CPU|386|in/out|N/A|extra GPIO to the head board|
+|LS_GPIO_CPU|391|in/out|N/A|extra GPIO to the low speed board|
+|STAY_ON_CPU|388|out|active high|pull high when flashing the low speed board to maintain power|
+