Add example code, and improve the facing docs slightly.

example/example.go

@@ -0,0 +1,6 @@
+// Serves as examples of using the goupnp library.
+//
+// To run examples and see the output for your local network, run the following
+// command (specifically including the -v flag):
+//     go test -v github.com/huin/goupnp/example
+package example

example/example_test.go

@@ -0,0 +1,48 @@
+package example_test
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/huin/goupnp/dcps/internetgateway1"
+)
+
+// Discovering a internet gateway devices on the local network, and asking each
+// of them for their external IP address.
+func Example_getExternalIPAddress() {
+	// import (
+	//   "fmt"
+	//   "os"
+	//   "github.com/huin/goupnp/dcps/internetgateway1"
+	// )
+	fmt.Println("Running")
+	clients, errors, err := internetgateway1.NewWANPPPConnection1Clients()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "Error discovering service with UPnP: ", err)
+		return
+	}
+
+	if len(errors) > 0 {
+		fmt.Fprintf(os.Stderr, "Error discovering %d services:\n", len(errors))
+		for _, err := range errors {
+			fmt.Println("  ", err)
+		}
+	}
+
+	fmt.Fprintf(os.Stderr, "Successfully discovered %d services:\n", len(clients))
+	for _, client := range clients {
+		device := &client.RootDevice.Device
+
+		fmt.Fprintln(os.Stderr, "  Device:", device.FriendlyName)
+		if addr, err := client.GetExternalIPAddress(); err != nil {
+			fmt.Fprintf(os.Stderr, "    Failed to get external IP address: %v\n", err)
+		} else {
+			fmt.Fprintf(os.Stderr, "    External IP address: %v\n", addr)
+		}
+	}
+	fmt.Println("Complete")
+
+	// Output:
+	// Running
+	// Complete
+}