Move English documentation to folder en_US

doc/en_US/plugin-overrides.markdown

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+Plugin Overrides
+================
+
+Override HTTP Content Security Policy
+-------------------------------------
+
+If you would like to replace the default HTTP Content Security Policy header, you can use the method `setContentSecurityPolicy()`:
+
+```php
+<?php
+
+namespace Kanboard\Plugin\Csp;
+
+use Kanboard\Core\Plugin\Base;
+
+class Plugin extends Base
+{
+    public function initialize()
+    {
+        $this->setContentSecurityPolicy(array('script-src' => 'something'));
+    }
+}
+```
+
+Template Overrides
+------------------
+
+Any templates defined in the core can be overridden. For example, you can redefine the default layout or change email notifications.
+
+Example of template override:
+
+```php
+$this->template->setTemplateOverride('header', 'theme:layout/header');
+```
+
+The first argument is the original template name and the second argument the template to use as replacement.
+
+You can still use the original template using the "kanboard:" prefix:
+
+```php
+<?= $this->render('kanboard:header') ?>
+```
+
+Formatter Overrides
+-------------------
+
+Here an example to override formatter objects in Kanboard:
+
+```php
+class MyFormatter extends UserAutoCompleteFormatter
+{
+    public function format()
+    {
+        $users = parent::format();
+
+        foreach ($users as &$user) {
+            $user['label'] = 'something'; // Do something useful here
+        }
+
+        return $users;
+    }
+}
+
+class Plugin extends Base
+{
+    public function initialize()
+    {
+        $this->container['userAutoCompleteFormatter'] = $this->container->factory(function ($c) {
+            return new MyFormatter($c);
+        });
+    }
+}
+```