Improve btcdwebsockets example.

Since the example illustrates callbacks for the OnBlockConnected and
OnBlockDisconnected callbacks, also register for the notifications with
NotifyBlocks.

While here, document the fact that most of the callbacks require
registration.

examples/btcdwebsockets/main.go

@@ -5,9 +5,9 @@
 package main
 
 import (
+	"github.com/conformal/btcrpcclient"
 	"github.com/conformal/btcutil"
 	"github.com/conformal/btcwire"
-	"github.com/conformal/btcrpcclient"
 	"io/ioutil"
 	"log"
 	"path/filepath"
@@ -16,6 +16,9 @@ import (
 
 func main() {
 	// Only override the handlers for notifications you care about.
+	// Also note most of these handlers will only be called if you register
+	// for notifications.  See the documentation of the btcrpcclient
+	// NotificationHandlers type for more details about each handler.
 	ntfnHandlers := btcrpcclient.NotificationHandlers{
 		OnBlockConnected: func(hash *btcwire.ShaHash, height int32) {
 			log.Printf("Block connected: %v (%d)", hash, height)
@@ -43,6 +46,12 @@ func main() {
 		log.Fatal(err)
 	}
 
+	// Register for block connect and disconnect notifications.
+	if err := client.NotifyBlocks(); err != nil {
+		log.Fatal(err)
+	}
+	log.Println("NotifyBlocks: Registration Complete")
+
 	// Get the current block count.
 	blockCount, err := client.GetBlockCount()
 	if err != nil {