Add link to Form API illustrations by Ricardo Sanz

See https://www.drupal.org/node/165104.
Blog post on https://metadrop.net/en/articles/form-api-drupalcon-prague.

Thanks to @rsanzante.

readme.md

@@ -120,8 +120,9 @@ Want to add/edit a link. Please follow the [Contribution guidelines](contributin
 - [Documentation](https://www.drupal.org/documentation)
 - [Core API Reference](https://api.drupal.org/api/drupal)
 - [Drupal 8.x+ APIs](https://www.drupal.org/node/2814041)
-- [Forms API](https://www.drupal.org/docs/drupal-apis/form-api)
-- [7.x Forms API reference](https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7.x)
+- [Form API](https://www.drupal.org/docs/drupal-apis/form-api)
+- [Form API Internal Workflow Illustration](https://www.drupal.org/node/165104)
+- [7.x Form API reference](https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7.x)
 
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