Queue up to 3 poll requests, Readme update

Using 3 queued poll requests we can be sure to not miss any change
by interrupt or manually triggered poll. See #59

README.md

@@ -200,8 +200,12 @@ doPoll (): Promise<void>;
 Manually poll changed inputs from the PCF8574/PCF8574A/PCF8575 IC.
 
 If a change on an input is detected, an `input` Event will be emitted with a data object containing the `pin` and the new `value`.
+
 This have to be called frequently enough if you don't use a GPIO for interrupt detection.
-If you poll again before the last poll was completed, the new poll will be queued up an get executed if the current poll is done.
+
+Poll requests will be queued internally up to 4 total requests (one active and three waiting) including polls triggered by interrupts.  
+If you try to trigger a poll while the queue is already full, the new poll will be rejected.  
+This is due to not have too many unnecessary polls.
 
 
 ### isPolling()

src/pcf857x.ts

@@ -125,7 +125,7 @@ export abstract class PCF857x<PinNumber extends PCF8574.PinNumber | PCF8575.PinN
   private _currentlyPolling: boolean = false;
 
   /** PromiseQueue to handle requested polls in order. */
-  private _pollQueue: PromiseQueue = new PromiseQueue(1);
+  private _pollQueue: PromiseQueue = new PromiseQueue(3);
 
   /** Pin number of GPIO to detect interrupts, or null by default. */
   private _gpioPin: number | null = null;