add readme info for cache

Author: docteurklein

README.markdown

@@ -58,6 +58,21 @@ $repositories = $client->api('user')->repositories('ornicar');
 
 From `$client` object, you can access to all GitHub.
 
+## Cache usage
+
+```php
+<?php
+
+// This file is generated by Composer
+require_once 'vendor/autoload.php';
+
+$client = new Github\Client(new CachedHttpClient(new FilesystemCache('/tmp/github-api-cache')));
+```
+
+Using cache, the client will get cached responses if resources haven't changed since last time,  
+**without** reaching the `X-Rate-Limit` [imposed by github](http://developer.github.com/v3/#rate-limiting).
+
+
 ## Documentation
 
 See the `doc` directory for more detailed documentation.