adds example for emptycallback closing #651

Has an explicit comment instructing the user in the use of `move` since thats something people struggle with. I could have made the example nicer and more functional. It could be refactored with a function that generates the callback. Then we would have something like:
```rust
fn new_playlist_pos_callback() -> impl Fn() + Send {
    ...
}

sink.append(song);
sink.append(new_playlist_pos_callback());
sink.append(song);
sink.append(new_playlist_pos_callback());
sink.append(song);
sink.append(new_playlist_pos_callback());
```

I chose instead to keep it as short and simple as possible, even though
the example code on its own is not as usefull now. Functions returning a generic boxed closure are hard to understand and that distracts from what we are trying to show here (how to use the emptycallback source).

examples/callback_on_end.rs

@@ -0,0 +1,41 @@
+use std::error::Error;
+use std::io::BufReader;
+use std::sync::atomic::{AtomicU32, Ordering};
+use std::sync::Arc;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let stream_handle = rodio::OutputStreamBuilder::open_default_stream()?;
+    let sink = rodio::Sink::connect_new(&stream_handle.mixer());
+
+    let file = std::fs::File::open("assets/music.wav")?;
+    sink.append(rodio::Decoder::new(BufReader::new(file))?);
+
+    // lets increment a number after `music.wav` has played. We are going to use atomics
+    // however you could also use a `Mutex` or send a message through a `std::sync::mpsc`.
+    let playlist_pos = Arc::new(AtomicU32::new(0));
+
+    // The closure needs to own everything it uses. We move a clone of
+    // playlist_pos into the closure. That way we can still access playlist_pos
+    // after appending the EmptyCallback.
+    let playlist_pos_clone = playlist_pos.clone();
+    sink.append(rodio::source::EmptyCallback::<f32>::new(Box::new(
+        move || {
+            println!("empty callback is now running");
+            playlist_pos_clone.fetch_add(1, Ordering::Relaxed);
+        },
+    )));
+
+    assert_eq!(playlist_pos.load(Ordering::Relaxed), 0);
+    println!(
+        "playlist position is: {}",
+        playlist_pos.load(Ordering::Relaxed)
+    );
+    sink.sleep_until_end();
+    assert_eq!(playlist_pos.load(Ordering::Relaxed), 1);
+    println!(
+        "playlist position is: {}",
+        playlist_pos.load(Ordering::Relaxed)
+    );
+
+    Ok(())
+}