add qetag example

Author: rwifeng

examples/qetag.php

@@ -0,0 +1,10 @@
+<?php
+require_once __DIR__ . '/../autoload.php';
+use Qiniu\Etag;
+
+list($etag, $err) = Etag::sum(__file__);
+if ($err == null) {
+	echo  "Etag: $etag";
+} else {
+	var_dump($err);
+}