Issue #2327095 by tstoeckler: Document how to allow test module and theme installation in example.settings.local.php.

sites/example.settings.local.php

@@ -37,6 +37,15 @@ $config['system.performance']['js']['preprocess'] = FALSE;
  */
 $settings['cache']['bins']['render'] = 'cache.backend.null';
 
+/**
+ * Allow test modules and themes to be installed.
+ *
+ * Drupal ignores test modules and themes by default for performance reasons.
+ * During development it can be useful to install test extensions for debugging
+ * purposes.
+ */
+$settings['extension_discovery_scan_tests'] = TRUE;
+
 /**
  * Enable access to rebuild.php.
  *