Add info about cache adapter to README. Added tests for cache and ftp

adapter.

README.markdown

@@ -338,3 +338,42 @@ knp_gaufrette:
                 prefix: APC 'namespace' prefix
                 ttl: 0
 ```
+
+## Cache 
+
+Adapter which allow to cache other adapters
+
+### Parameters
+
+ * `source` The source adapter that must be cached *(required)*
+ * `cache` The adapter used to cache the source *(required)*
+ * `ttl` Time to live *(default 0)*
+ * `serializer` The adapter used to cache serializations *(default null)*
+
+### Example
+
+``` yaml
+# app/config/config.yml
+knp_gaufrette:
+    adapters:
+        media_ftp:
+            ftp:
+                host: example.com
+                username: user
+                password: pass
+                directory: /example/ftp
+                create: true
+                mode: FTP_BINARY
+        media_apc:
+            apc:
+                prefix: APC 'namespace' prefix
+                ttl: 0
+        media_cache:
+            cache:
+                source: media_ftp
+                cache: media_apc
+                ttl: 7200
+    filesystems:
+        media:
+            adapter: media_cache
+```