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

Author: komainu8

sample/document-create-document-type.lua

@@ -0,0 +1,13 @@
+#!/usr/bin/env luajit
+
+local xmlua = require("xmlua")
+
+--Specify a location by PublicID
+local document = xmlua.XML.build({})
+local document_type =
+  document:create_document_type("TestDocumentDecl",
+                                "-//Test/Sample/EN",
+                                "//test.dtd")
+print(document:to_xml())
+--<?xml version="1.0" encoding="UTF-8"?>
+--<!DOCTYPE TestDocumentDecl PUBLIC "-//Test/Sample/EN" "//system.dtd">