Add comment with link to PORT_PCR_ISF polling example

teensy3/kinetis.h

@@ -870,6 +870,7 @@ enum IRQ_NUMBER_t {
 
 #define PORTA_PCR0		(*(volatile uint32_t *)0x40049000) // Pin Control Register n
 #define PORT_PCR_ISF			((uint32_t)0x01000000)		// Interrupt Status Flag
+	// how to use PORT_PCR_ISF with polling: https://forum.pjrc.com/threads/58193
 #define PORT_PCR_IRQC(n)		((uint32_t)(((n) & 15) << 16))	// Interrupt Configuration
 #define PORT_PCR_IRQC_MASK		((uint32_t)0x000F0000)
 #define PORT_PCR_LK			((uint32_t)0x00008000)		// Lock Register