Update the list of integrations

The integrations are now organized by the name of the integration, and
themes are now separated out from the plugins. The references for this
section have been pulled out at the bottom because there are so many.

This closes https://github.com/select2/select2/issues/4150.
This closes https://github.com/select2/select2/pull/4067.
This closes https://github.com/select2/select2/issues/3957.

README.md

@@ -39,19 +39,25 @@ the integrations below.
 
 Integrations
 ------------
-* [Wicket-Select2][wicket-select2] (Java / [Apache Wicket][wicket])
-* [select2-rails][select2-rails] (Ruby on Rails)
-* [AngularUI][angularui-select] ([AngularJS][angularjs])
-* [Django][django-select2]
-* [Symfony][symfony-select2]
-* [Symfony2][symfony2-select2]
-* [Bootstrap 2][bootstrap2-select2] and [Bootstrap 3][bootstrap3-select2]
-  (CSS skins)
-* [Meteor][meteor-select2] ([Bootstrap 3 skin][meteor-select2-bootstrap3])
-* [Meteor][meteor-select2-alt]
-* [Yii 2.x][yii2-select2]
-* [Yii 1.x][yii-select2]
-* [AtmosphereJS][atmospherejs-select2]
+Third party developers have create plugins for platforms which allow Select2 to be integrated more natively and quickly. For many platforms, additional plugins are not required because Select2 acts as a standard `<select>` box.
+
+Plugins
+
+* [Django]
+  - [django-easy-select2]
+  - [django-select2]
+* [Meteor] - [meteor-select2]
+* [Ruby on Rails][ruby-on-rails] - [select2-rails]
+* [Wicket] - [wicketstuff-select2]
+* [Yii 2][yii2] - [yii2-widget-select2]
+
+Themes
+
+- [Bootstrap 3][bootstrap3] - [select2-bootstrap-theme]
+- [Flat UI][flat-ui] - [select2-flat-theme]
+- [Metro UI][metro-ui] - [select2-metro]
+
+Missing an integration? Modify this `README` and make a pull request back here to Select2 on GitHub.
 
 Internationalization (i18n)
 ---------------------------
@@ -77,29 +83,33 @@ Copyright and license
 ---------------------
 The license is available within the repository in the [LICENSE][license] file.
 
-[angularjs]: https://angularjs.org/
-[angularui-select]: http://angular-ui.github.io/#ui-select
-[atmospherejs-select2]: https://atmospherejs.com/package/jquery-select2
-[bootstrap2-select2]: https://github.com/t0m/select2-bootstrap-css
-[bootstrap3-select2]: https://github.com/t0m/select2-bootstrap-css/tree/bootstrap3
 [cdnjs]: http://www.cdnjs.com/libraries/select2
 [community]: https://select2.github.io/community.html
-[django-select2]: https://github.com/applegrew/django-select2
 [documentation]: https://select2.github.io/
 [documentation-folder]: https://github.com/select2/select2/tree/master/docs
 [freenode]: https://freenode.net/
 [jsdelivr]: http://www.jsdelivr.com/#!select2
 [license]: LICENSE.md
-[meteor-select2]: https://github.com/nate-strauser/meteor-select2
-[meteor-select2-alt]: https://jquery-select2.meteor.com
-[meteor-select2-bootstrap3]: https://github.com/zimme/meteor-select2-bootstrap3-css/
 [releases]: https://github.com/select2/select2/releases
-[select2-rails]: https://github.com/argerim/select2-rails
-[symfony-select2]: https://github.com/19Gerhard85/sfSelect2WidgetsPlugin
-[symfony2-select2]: https://github.com/avocode/FormExtensions
 [travis-ci-image]: https://travis-ci.org/select2/select2.svg?branch=master
 [travis-ci-status]: https://travis-ci.org/select2/select2
-[wicket]: http://wicket.apache.org
-[wicket-select2]: https://github.com/ivaynberg/wicket-select2
-[yii-select2]: https://github.com/tonybolzan/yii-select2
-[yii2-select2]: http://demos.krajee.com/widgets#select2
+
+[bootstrap3]: https://getbootstrap.com/
+[django]: https://www.djangoproject.com/
+[django-easy-select2]: https://github.com/asyncee/django-easy-select2
+[django-select2]: https://github.com/applegrew/django-select2
+[flat-ui]: http://designmodo.github.io/Flat-UI/
+[meteor]: https://www.meteor.com/
+[meteor-select2]: https://github.com/nate-strauser/meteor-select2
+[metro-ui]: http://metroui.org.ua/
+[select2-metro]: http://metroui.org.ua/select2.html
+[ruby-on-rails]: http://rubyonrails.org/
+[select2-bootstrap-theme]: https://github.com/select2/select2-bootstrap-theme
+[select2-flat-theme]: https://github.com/techhysahil/select2-Flat_Theme
+[select2-rails]: https://github.com/argerim/select2-rails
+[vue.js]: http://vuejs.org/
+[select2-vue]: http://vuejs.org/examples/select2.html
+[wicket]: https://wicket.apache.org/
+[wicketstuff-select2]: https://github.com/wicketstuff/core/tree/master/select2-parent
+[yii2]: http://www.yiiframework.com/
+[yii2-widget-select2]: https://github.com/kartik-v/yii2-widget-select2