- cd /path/to/moodle/lib/editor/tinymce/plugins
- git clone https://github.com/cleblond/moodle-tinymce_rj_insertcode.git
- mv moodle-tinymce_rj_insertcode rj_insertcode
- Remove the ampersand from line 58 of rj_insertcode/php/rj_get_highlighted_code.php because PHP5.
- On the TinyMCE settings page, add "|,rj_insertcode" to Editor Toolbar.
- Purge cache.
It is not clear that adding the plugin to lib.php is necessary. I did, then undid, it.
As shown below, my Settings page doesn't look pretty, but I can live with that (or revisit later).