Mention example in README.

Author: dlevy47

README.md

@@ -1,3 +1,30 @@
 # Rust bindings to retrieve network interface information
 
 This library contains functionality to retrieve network interface information on Linux machines.
+
+## Example usage
+
+See `src/bin/ifaces.rs`:
+
+```rust
+extern crate "rust-ifaces" as ifaces;
+
+fn main () {
+    for iface in
+        ifaces::Interface::get_all().unwrap()
+            .into_iter()
+            .filter(|x| x.kind == ifaces::Kind::Packet) {
+                println!("{}", iface.name);
+            }
+}
+```
+
+On my machine, this prints out:
+
+```
+$ target/ifaces
+lo
+eth0
+wlan0
+docker0
+```