Added info about SetIcon on menu items to readme and example

README.md

@@ -13,6 +13,9 @@ func onReady() {
 	systray.SetTitle("Awesome App")
 	systray.SetTooltip("Pretty awesome超级棒")
 	mQuit := systray.AddMenuItem("Quit", "Quit the whole app")
+
+	// Sets the icon of a menu item. Only available on Mac.
+	mQuit.SetIcon(icon.Data)
 }
 
 func onExit() {

example/main.go

@@ -44,6 +44,10 @@ func onReady() {
 		systray.AddMenuItem("Ignored", "Ignored")
 		mUrl := systray.AddMenuItem("Open Lantern.org", "my home")
 		mQuit := systray.AddMenuItem("退出", "Quit the whole app")
+
+		// Sets the icon of a menu item. Only available on Mac.
+		mQuit.SetIcon(icon.Data)
+
 		systray.AddSeparator()
 		mToggle := systray.AddMenuItem("Toggle", "Toggle the Quit button")
 		shown := true