Move English documentation to folder en_US

doc/en_US/plugin-events.markdown

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+Using Events
+============
+
+Kanboard use internally the [Symfony EventDispatcher component](https://symfony.com/doc/2.3/components/event_dispatcher/index.html) to manage internal events.
+
+Event Listening
+---------------
+
+```php
+$this->on('app.bootstrap', function($container) {
+    // Do something
+});
+```
+
+- The first argument is the event name (string)
+- The second argument is a PHP callable function (closure or class method)
+
+Adding a new event
+------------------
+
+To add a new event, you have to call the method `register()` of the class `Kanboard\Core\Event\EventManager`:
+
+```php
+$this->eventManager->register('my.event.name', 'My new event description');
+```
+
+These events can be used by other components of Kanboard like automatic actions.