Install Alert Messages plugin to CKEditor

Unpack archived files directly to ckeditor/plugins.
Turn on the plugin in ckeditor/config.js:
```
config.extraPlugins = 'jsplus_alert';
```
Add these buttons to the toolbar definition:
- jsplus_alert
  Insert information message
See sample CKEditor toolbar configuration to find out more about this step.
Now you can use your installed plugins. Do not forget to sure that browser cache was cleared.