sample: add sample for "Document:create_namespace()"

Author: komainu8

sample/document-create-namespace.lua

@@ -0,0 +1,16 @@
+#!/usr/bin/env luajit
+
+local xmlua = require("xmlua")
+
+local document = xmlua.XML.build({"root"})
+local namespace =
+  document:create_namespace("http://www.w3.org/1999/xhtml",
+                            "xhtml")
+local root = document:root()
+root:set_namespace(namespace)
+print(namespace:href(), namespace:prefix(), document:to_xml())
+--"http://www.w3.org/1999/xhtml",
+--"xhtml",
+--<?xml version="1.0" encoding="UTF-8"?>
+--<xhtml:root/>
+