If you want to use a plugin, you have to include the corresponding file.
In app/assets/javascripts/application.js
, you should add the following:
// load hello plugin
//= require summernote/plugin/hello/summernote-ext-hello
Then, update Summernote's initialization options:
$(function () {
'use strict';
$('[data-provider="summernote"]').each(function () {
$(this).summernote({
height: 300,
toolbar : [
// ...
['insert', ['hello']]
]
});
});