Create SECURITY.md (#3757)

* Create SECURITY.md

Adds a GitHub security policy, still in beta but will be presumably standard in the future.

* Added License header

SECURITY.md

@@ -0,0 +1,33 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| < 2.2   | :x:                |
+| 2.2.x   | :white_check_mark: |
+| 3.0.x   | :white_check_mark: |
+
+## Reporting a Vulnerability
+Contact the [Apache Software Foundation Security Team](mailto:security@trafficcontrol.apache.org)
+and follow the guidelines on [the Apache Software Foundation security page](https://www.apache.org/security/)
+regarding vulnerability disclosure.
+
+<!--
+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.
+-->