Add file header description to index.js

src/index.js

@@ -1,3 +1,20 @@
+/**
+ * This file is part of Mondo.
+ * 
+ * Mondo 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.
+ * 
+ * Mondo 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 Mondo.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
 const client = require('discord-rich-presence')('763579990209855559')
 const { app, BrowserWindow, ipcMain, dialog } = require('electron')
 const { autoUpdater } = require('electron-updater')