Add license header (#720)

Author: Ace Nassri

functions/firebase/index.js

@@ -1,3 +1,18 @@
+/**
+ * Copyright 2018, Google LLC.
+ * 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.
+ */
+
 // [START functions_firebase_rtdb]
 /**
  * Triggered by a change to a Firebase RTDB reference.