SECURITY.md: add the ASF license header instead of excluding it from RAT

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: leerho

.rat-excludes

@@ -16,7 +16,6 @@ fdlibm_log\.hpp
 README\.md
 CONTRIBUTING\.md
 CODE_OF_CONDUCT\.md
-SECURITY\.md
 #
 # Generated and template files consumed by the build.
 Doxyfile

SECURITY.md

@@ -1,3 +1,22 @@
+<!--
+    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.
+-->
+
 # Security Policy
 
 ## Reporting a Vulnerability