add example for providing own FileNameGenerator

Author: danikula

README.md

@@ -8,6 +8,7 @@
 - [Recipes](#recipes)
   - [Disk cache limit](#disk-cache-limit)
   - [Listen caching progress](#listen-caching-progress)
+  - [Providing names for cached files](#providing-names-for-cached-files)
   - [Sample](#sample)
 - [Known problems](#known-problems)
 - [Whats new](#whats-new)
@@ -105,6 +106,26 @@ Use `HttpProxyCacheServer.isCached(String url)` method to check was url's conten
 
 See `sample` app for more details.
 
+### Providing names for cached files
+By default `AndroidVideoCache` uses MD5 of video url as file name. But in some cases url is not stable and it can contain some generated parts (e.g. session token). In this case caching mechanism will be broken. To fix it you have to provide own `FileNameGenerator`:
+``` java
+public class MyFileNameGenerator implements FileNameGenerator {
+
+    // Urls contain mutable parts (parameter 'sessionToken') and stable video's id (parameter 'videoId').
+    // e. g. http://example.com?videoId=abcqaz&sessionToken=xyz987
+    public String generate(String url) {
+        Uri uri = Uri.parse(url);
+        String videoId = uri.getQueryParameter("videoId");
+        return videoId + ".mp4";
+    }
+}
+
+...
+HttpProxyCacheServer proxy = HttpProxyCacheServer.Builder(context)
+    .fileNameGenerator(new MyFileNameGenerator())
+    .build()
+```
+
 ### Sample
 See `sample` app.