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.

Cargo.toml

@@ -50,6 +50,7 @@ futures = "0.3"
 serde_json = "1.0.39"
 tempfile = "3.2.0"
 nix = "0.23.1"
+glob = "0.3.0"
 
 [features]
 default = ["macos_fsevent"]

examples/poll_sysfs.rs

@@ -0,0 +1,43 @@
+use std::path::Path;
+use std::time::Duration;
+use notify::poll::PollWatcherConfig;
+use notify::{PollWatcher, RecursiveMode, Watcher};
+use glob::glob;
+
+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 interfaces = glob("/sys/class/net/*/statistics")
+        .map_err(|e| notify::Error::generic(&e.to_string()))?
+        .filter_map(|result| result.ok());
+    paths.extend(interfaces);
+  }
+
+  if paths.is_empty() {
+    eprintln!("Must provide path to watch, default system paths in /sys/class/net were not found (maybe you're not running on Linux?)");
+    std::process::exit(1);
+  }
+
+  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(())
+}