updating contributin doc (#8)

Author: aliabbasrizvi

CONTRIBUTING.md

@@ -17,7 +17,27 @@ We welcome contributions and feedback! All contributors must sign our [Contribut
 
 ##License
 
-By contributing your code, you agree to license your contribution under the terms of the [Apache License v2.0](http://www.apache.org/licenses/LICENSE-2.0).
+All contributions are under the CLA mentioned above. For this project, Optimizely uses the Apache 2.0 license, and so asks that by contributing your code, you agree to license your contribution under the terms of the [Apache License v2.0](http://www.apache.org/licenses/LICENSE-2.0). Your contributions should also include the following header:
+
+```
+/**
+ * Copyright YEAR, Optimizely, Inc. and contributors
+ *
+ * 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.
+ */
+```
+
+The YEAR above should be the year of the contribution. If work on the file has been done over multiple years, list each year in the section above. Example: Optimizely writes the file and releases it in 2014. No changes are made in 2015. Change made in 2016. YEAR should be “2014, 2016”.
 
 ##Contact
 If you have questions, please contact [email protected].