Update readme

README.md

@@ -17,3 +17,6 @@ $entity->metadata->set("name", "PCM");
 $entity->metadata->get("name"); // "PCM"
 $entity->metadata->isSet("name"); // true
 ```
+
+> Like setting a property normally, setting metadata values will not automatically update the database.
+You must call `flush()` from the `EntityManager` as usual when you wish to persist data.