Described how to set up persistent disk cache

This is good enough "fix" for issue hub4j#168.

src/site/markdown/index.md

@@ -41,4 +41,11 @@ This library comes with a pluggable connector to use different HTTP client imple
 through `HttpConnector`. In particular, this means you can use [OkHttp](http://square.github.io/okhttp/),
 so we can make use of it's HTTP response cache.
 Making a conditional request against the GitHub API and receiving a 304 response
-[does not count against the rate limit](http://developer.github.com/v3/#conditional-requests).
+[does not count against the rate limit](http://developer.github.com/v3/#conditional-requests).
+
+The following code shows an example of how to set up persistent cache on the disk:
+
+    Cache cache = new Cache(cacheDirectory, 10 * 1024 * 1024); // 10MB cache
+    GitHub gitHub = GitHubBuilder.fromCredentials()
+        .withConnector(new OkHttpConnector(new OkUrlFactory(new OkHttpClient().setCache(cache))))
+        .build();