Update config.php.sample

Added cookie variables to allow wildcard cookies to be set for the domain by other non-PHP web apps on the same TLD, or set a path for the cookie domain if required to be different than '/'

rootfs/flarum/app/config.php.sample

@@ -18,4 +18,8 @@
     'api' => 'api',
     'admin' => 'admin',
   ),
+  'cookie' => array (
+    'path' => '<COOKIE_PATH>',
+    'domain' => '<COOKIE_DOMAIN>',
+  ),
 );