Add licensing notice

src/extension.js

@@ -1,3 +1,21 @@
+/*
+Night Theme Switcher Gnome Shell extension
+
+Copyright (C) 2019 Romain Vigier
+
+This program is free software: you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free Software
+Foundation, either version 3 of the License, or (at your option) any later
+version.
+
+This program is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program. If not, see <http s ://www.gnu.org/licenses/>.
+*/
+
 'use strict';
 
 const ExtensionUtils = imports.misc.extensionUtils;