How can I add Zurb Foundation as a library?
You created the library, alright, but you need to call the library.

In [theme_name].info.yml add the following:

libraries:
  - '[theme_name]/foundation'


Also you need to modify your [theme_name].libraries.yml file and add dependencies:

foundation:
  version: 6.x
  css:
    base:
      includes/foundation/css/app.css: {}
      includes/foundation/css/foundation.css: {}
      includes/foundation/css/foundation.min.css: {}
  js:
    includes/foundation/js/app.js: {}
    includes/foundation‌​/js/jquery.js: {}
    includes/foundation/js/foundation.js: {}
    includes/foundation/js/foundation.min.js: {}
    includes/foundation/js/what-input.js: {}
  dependencies:
    - core/drupal
    - core/jquery
    - core/jquery.once


As per Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 theme