Add comment to cache file driver.

system/cache/file.php

@@ -29,6 +29,8 @@ class File implements Driver {
 
 		$cache = file_get_contents(CACHE_PATH.$key);
 
+		// The cache expiration date is stored as a UNIX timestamp at the beginning
+		// of the cache file. We'll extract it out and check it here.
 		if (time() >= substr($cache, 0, 10))
 		{
 			$this->forget($key);