I need to install extra plugins such as: youtube, table of contents and other. Download plugin from official web site https://ckeditor.com/cke4/addon/youtube put in directory /mysite/core/modules/ckeditor/js/plugins and enable the plugin by changing or adding the extraPlugins line in your configuration (config.js):
config.extraPlugins = 'youtube';
But I didn't found file config.js. Where I must add this string, for getting these plugins work?