Add routes file to MailchimpPlugin to handle custom routes

Signed-off-by: objec <objecttothis@gmail.com>

app/Plugins/MailchimpPlugin/Config/Routes.php

@@ -0,0 +1,8 @@
+<?php
+
+use CodeIgniter\Router\RouteCollection;
+
+/**
+ * @var RouteCollection $routes
+ */
+$routes->post('plugins/mailchimp/checkMailchimpApiKey', '\App\Plugins\MailchimpPlugin\Controllers\MailchimpController::postCheckMailchimpApiKey');