add src header

Author: etpinard

src/components/legend/constants.js

@@ -1,3 +1,11 @@
+/**
+* Copyright 2012-2016, Plotly, Inc.
+* All rights reserved.
+*
+* This source code is licensed under the MIT license found in the
+* LICENSE file in the root directory of this source tree.
+*/
+
 'use strict';
 
 module.exports = {