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The process for defining new rich text features is documented immediately below the section you linked to:
http://docs.wagtail.io/en/v1.12/advanced_topics/customisation/page_editing_interface.html#extending-the-wysiwyg-editor-hallo-js
I would strongly recommend NOT enabling underline and justification options, though… Wagtail leaves them out for good reason. On the web, underline indicates a link – using underline for other things will look ugly and annoy your users.
Justification is a design choice, and design should be handled within the HTML and CSS, not by editors within the CMS. Editors should be dealing with the information content, not the presentation.
Source:stackexchange.com