reference MAGPIE_CACHE_AGE global config option

config.php-dist

@@ -18,6 +18,10 @@
 	define('MAGPIE_CACHE_DIR', '/var/tmp/magpie-ttrss-cache');
 	// Local cache directory for RSS feeds
 
+	define('MAGPIE_CACHE_AGE', 60*30); 
+	// How long to store cached RSS objects? In seconds.
+	// Defaults to 30 minutes
+
 	define('WEB_DEMO_MODE', false);
 	// Demo mode with reduced functionality.
 	// Warning: this option is obsolete. Don't rely on it being