Update CONTRIBUTING.md with license header (ExpediaGroup#348)

Author: smyrick

CONTRIBUTING.md

@@ -55,6 +55,24 @@ mvn antrun:run@detekt
 
 Please add appropriate javadocs in the source code and ask the maintainers to update the wiki with any relevant information.
 
+## Add license information
+All source files must contain the following license header. If you are using an IDE please add this as a copyright template for this project so that it will be added automatically.
+
+```
+Copyright $today.year Expedia, Inc
+
+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.
+ ```
 
 ## Merging your contribution