fix: add explanation to example

At the moment the example screen is black, and user does not know what he is supposed to do.  Add instruction what user action will trigger playing the sound effect.

Author: elgopher

_examples/audio/lowlevel/main.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/elgopher/pi"
 	"github.com/elgopher/pi/piaudio"
+	"github.com/elgopher/pi/picofont"
 	"github.com/elgopher/pi/piebiten"
 	"github.com/elgopher/pi/pievent"
 	"github.com/elgopher/pi/piloop"
@@ -25,6 +26,8 @@ var clickWav []byte
 func main() {
 	sample := piaudio.DecodeWav(clickWav)
 
+	picofont.Print("PRESS LEFT MOUSE BUTTON TO PLAY SFX", 90, 80)
+
 	pi.Init = func() {
 		// The sample must be loaded before use,
 		// but communication with the audio backend is only possible after starting the game.