Update README.md

Plugin options
url, pageTitle - parameters to be passed to Google Tag manager
triggerDelay - fire up a virtual page after some time (to make sure user is not just scrolling)
downPercentage, upPercentage - percentage the "page" takes from the whole height when it triggers the event (for example, downPercentage:0.6 - on the way down trigger event when the "page" takes 60% of the viewport height)