Add Apache License header to S3 Conditional Requests documentation

Author: peterxcli

hadoop-hdds/docs/content/design/s3-conditional-requests.md

@@ -6,6 +6,19 @@ jira: HDDS-13117
 status: draft
 author: Chu Cheng Li
 ---
+<!--
+  Licensed 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. See accompanying LICENSE file.
+-->
 
 # S3 Conditional Requests Design