Improve addCurrentTimeChangeListener documentation

[javier-godoy]

Mention that addCurrentTimeChangeListener configures throttling. See #24

SimpleTimerAddon/src/main/java/com/flowingcode/vaadin/addons/simpletimer/SimpleTimer.java

Lines 160 to 166 in 8db1c45

	/**
	* Adds a property change listener for the {@code currentTime} property
	*
	* @return current value in seconds
	*/
	public Registration addCurrentTimeChangeListener(
	PropertyChangeListener listener, long time, TimeUnit timeUnit) {