Add example of new PollWatcher compare_contents feature

The example shows how to use this to effectively watch pseudo
filesystems like those through sysfs (i.e. /sys/).  The example
by default will only work on Linux but does demonstrate well that it
works where the previous metadata only approach would not.

Sample output:

  Trying "/sys/class/net/lo/statistics/tx_bytes", use `ping localhost` to see changes!
  watching ["/sys/class/net/lo/statistics/tx_bytes"]...
  changed: Event { kind: Modify(Data(Any)), paths: ["/sys/class/net/lo/statistics/tx_bytes"], attr:tracker: None, attr:flag: None, attr:info: None, attr:source: None }
  changed: Event { kind: Modify(Data(Any)), paths: ["/sys/class/net/lo/statistics/tx_bytes"], attr:tracker: None, attr:flag: None, attr:info: None, attr:source: None }
  changed: Event { kind: Modify(Data(Any)), paths: ["/sys/class/net/lo/statistics/tx_bytes"], attr:tracker: None, attr:flag: None, attr:info: None, attr:source: None }
  changed: Event { kind: Modify(Data(Any)), paths: ["/sys/class/net/lo/statistics/tx_bytes"], attr:tracker: None, attr:flag: None, attr:info: None, attr:source: None }

examples/poll_sysfs.rs

@@ -0,0 +1,40 @@
+use std::path::Path;
+use std::time::Duration;
+use notify::poll::PollWatcherConfig;
+use notify::{PollWatcher, RecursiveMode, Watcher};
+
+fn main() -> notify::Result<()> {
+  let mut paths: Vec<_> = std::env::args().skip(1)
+      .map(|arg| Path::new(&arg).to_path_buf())
+      .collect();
+  if paths.is_empty() {
+    let lo_stats = Path::new("/sys/class/net/lo/statistics/tx_bytes").to_path_buf();
+    if !lo_stats.exists() {
+      eprintln!("Must provide path to watch, default system path was not found (probably you're not running on Linux?)");
+      std::process::exit(1);
+    }
+    println!("Trying {lo_stats:?}, use `ping localhost` to see changes!");
+    paths.push(lo_stats);
+  }
+
+  println!("watching {:?}...", paths);
+
+  let config = PollWatcherConfig {
+    compare_contents: true,
+    poll_interval: Duration::from_secs(2),
+  };
+  let (tx, rx) = std::sync::mpsc::channel();
+  let mut watcher = PollWatcher::with_config(tx, config)?;
+  for path in paths {
+    watcher.watch(&path, RecursiveMode::Recursive)?;
+  }
+
+  for res in rx {
+    match res {
+      Ok(event) => println!("changed: {:?}", event),
+      Err(e) => println!("watch error: {:?}", e),
+    }
+  }
+
+  Ok(())
+}