Add Apache License sample header

CONTRIBUTING.adoc

@@ -47,3 +47,25 @@ Please don't report security vulnerabilities using GitHub issues, instead head o
 Before we accept a non-trivial patch or pull request we will need you to https://cla.pivotal.io/sign/spring[sign the Contributor License Agreement].
 Signing the contributor's agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do.
 Active contributors might be asked to join the core team, and given the ability to merge pull requests.
+
+== Apache License header
+
+Please add the Apache License header to all new classes, for example:
+
+```java
+/*
+ * Copyright 2020 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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.
+ */
+```