Update readme

Author: snapshotpl

README.md

@@ -1,2 +1,10 @@
-# maintenance
+# maintenance [![Build Status](https://travis-ci.org/php-middleware/maintenance.svg?branch=master)](https://travis-ci.org/php-middleware/maintenance)
 Site maintenance middleware SEO friendly
+
+## Features
+
+* Setup 503 header,
+* Support `Retry-After` header (as seconds, or HTTP-date),
+* Support `Redirect` header (redirect page after `Retry-After` time).
+
+More about this SEO practice: [How to deal with planned site downtime](http://googlewebmastercentral.blogspot.com/2011/01/how-to-deal-with-planned-site-downtime.html) in Google Webmaster Central Blog.