See the documentation at https://www.drupal.org/developing/api/8/ckeditor.
You can find examples in Drupal 8 core, at:
\Drupal\ckeditor_test\Plugin\CKEditorPlugin\Llama\Drupal\ckeditor_test\Plugin\CKEditorPlugin\LlamaButton\Drupal\ckeditor_test\Plugin\CKEditorPlugin\LlamaContextual\Drupal\ckeditor_test\Plugin\CKEditorPlugin\LlamaContextualAndButton
(This is the exact same answer as I gave at http://drupal.stackexchange.com/a/191869/7632.)
To answer specifically why the button is not showing: CKEditor's Advanced Content Filter (ACF).
Please add your button to CKEditor in the "Full HTML" text format. Check if it shows up then. Chances are it does.
Your plugin does not specify ACF metadata. ACF is what ensures CKEditor produces clean markup. In "Full HTML", there are no restrictions, and no guarantees of clean markup.