chore: bootstrap module from working instance and add install guide

notifications/DonationNotificationCategory.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace humhub\modules\donations\notifications;
+
+use humhub\modules\notification\components\NotificationCategory;
+use Yii;
+
+class DonationNotificationCategory extends NotificationCategory
+{
+    public $id = 'donations_activity';
+
+    public function getTitle()
+    {
+        return Yii::t('DonationsModule.base', 'Donations');
+    }
+
+    public function getDescription()
+    {
+        return Yii::t('DonationsModule.base', 'Receive notifications about successful or refunded donations.');
+    }
+}