add comment on broken test

Author: l-trotta

java-client/src/test/java/co/elastic/clients/elasticsearch/model/SerializationTest.java

@@ -41,6 +41,7 @@
 
 public class SerializationTest extends ModelTestCase {
 
+    // TODO Jackson 3 doesn't work
     @Test
     public void loadAllDeserializers() throws Exception {