Added a page describing how I use GPIOs to instrument some code.

Instrumenting-code-using-GPIOs.md

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+I often find that I want to instrument things (like ISR routines) so I get some detailed timing information. So I thought I would document the process I use.
+
+###Initialize the Pins###
+
+I typically use pins X1, X2, X3, and X4, because they happen to correspond to CPU pins A0, A1, A2, and A3. We need to initialize the pins as outputs. A reasonable place to put the initialization code is in main.c, just after the GPIO clocks are initialized (search for ```__GPIOD_CLK_ENABLE```). The following snippet is what I would use to initialize 4 pins:
+```C
+    GPIO_InitTypeDef GPIO_InitStructure;
+    GPIO_InitStructure.Mode = GPIO_MODE_OUTPUT_PP;
+    GPIO_InitStructure.Pull = GPIO_NOPULL;
+    GPIO_InitStructure.Speed = GPIO_SPEED_HIGH;
+    GPIO_InitStructure.Alternate = 0;
+    GPIO_InitStructure.Pin = GPIO_PIN_0 | GPIO_PIN_1 | GPIO_PIN_2 | GPIO_PIN_3;
+    HAL_GPIO_Init(GPIOA, &GPIO_InitStructure);
+```
+main.c already includes enough headers that nothing extra is needed for the above.
+
+###Toggle the pins###
+
+Let's suppose that I want to instrument the IRQ Handler for UART6. In the ```stm32_it.c``` file, we'll need to pull in some header files. So add the following #includes:
+```C
+#include "py/mphal.h"
+#include "pin.h"
+#include "genhdr/pins.h"
+```
+and now we can toggle a GPIO in the ISR handler like so:
+```C
+void USART6_IRQHandler(void) {
+    GPIO_set_pin(pin_A0.gpio, pin_A0.pin_mask);
+    uart_irq_handler(6);
+    GPIO_clear_pin(pin_A0.gpio, pin_A0.pin_mask);
+}
+```
+
+###Capture###
+Here's what a typical capture might look like. This was done using the above code and captured while receiving UART data at 1 Mbit/sec.
+![UART6 1 Mbit/sec capture](https://www.dropbox.com/s/2gnnh9o2rsis4bz/UART-1Mbit.png?dl=0)