proper licensing

packages/flutter_mozilla_components/javascript/readability/src/index.js

@@ -1,3 +1,9 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
 import { Readability, isProbablyReaderable } from '@mozilla/readability';
 import TurndownService from 'turndown';