added comment regarding nested route directories.

system/router.php

@@ -73,6 +73,9 @@ class Router {
 	{
 		$segments = explode('/', $uri);
 
+		// Route files can be nested deep within sub-directories. To find the
+		// appropriate file, we work our way backward through the URI looking
+		// for the deepest matching file.
 		foreach (array_reverse($segments, true) as $key => $value)
 		{
 			if (file_exists($path = APP_PATH.'routes/'.implode('/', array_slice($segments, 0, $key + 1)).EXT))