fix input logic (pull-up) + add GPIO notes

LED Blink Input Output/main.c

@@ -14,7 +14,7 @@ int main(void) {
 
   while(1) {
     // Button pressed -> pin reads LOW because of pull-up
-    if (PINB & (1<<PB3)) {
+    if (!(PINB & (1<<PB3))) {
       for (int i=0;i<=20;i++){
       PORTB |= (1<<PB2);      // LED ON
       delay_ms(200);