Add missing license header for md files (#18541) (#19189)

Co-authored-by: ciyong <[email protected]>

R-package/README.md

@@ -1,3 +1,20 @@
+<!--- Licensed to the Apache Software Foundation (ASF) under one -->
+<!--- or more contributor license agreements.  See the NOTICE file -->
+<!--- distributed with this work for additional information -->
+<!--- regarding copyright ownership.  The ASF licenses this file -->
+<!--- to you under the Apache License, Version 2.0 (the -->
+<!--- "License"); you may not use this file except in compliance -->
+<!--- with the License.  You may obtain a copy of the License at -->
+<!--- -->
+<!---   http://www.apache.org/licenses/LICENSE-2.0 -->
+<!--- -->
+<!--- Unless required by applicable law or agreed to in writing, -->
+<!--- software distributed under the License is distributed on an -->
+<!--- "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY -->
+<!--- KIND, either express or implied.  See the License for the -->
+<!--- specific language governing permissions and limitations -->
+<!--- under the License. -->
+
 <img src=https://raw.githubusercontent.com/dmlc/dmlc.github.io/master/img/logo-m/mxnetR.png width=155/> Deep Learning for R
 ==========================