How do I add extra/custom plugins in CKEditor? I am copying the templates plugin http://ckeditor.com/addon/templates 
I put    it in my D8 folder /core/asstes/vendor/ckeditor/plugins
I do a 'drush    cr'


I do not see the templates plugin under available buttons in my text format

How do I get the templates plugin to show up?
You have to build a plugin in Drupal using the CKEditor classes in the CKEditor module in core, it is not an automatic integration. 

Check some of the existing modules that integrate plugins for examples on how to do this. 

Checkout:


CodeSnippet
Color Button
Panel Button