[docs] clients need to add jackson-databind (#21527)

With ES 5.0 we do not include Jackson
Databind anymore with ES core. This commit
updates our docs to state that users need
to add this artifact now in their projects.

docs/java-api/docs/index_.asciidoc

@@ -60,8 +60,9 @@ json.put("message","trying out Elasticsearch");
 [[java-docs-index-generate-beans]]
 ===== Serialize your beans
 
-Elasticsearch already uses http://wiki.fasterxml.com/JacksonHome[Jackson].
-So you can use it to serialize your beans to JSON:
+You can use http://wiki.fasterxml.com/JacksonHome[Jackson] to serialize
+your beans to JSON. Please add http://search.maven.org/#search%7Cga%7C1%7Cjackson-databind[Jackson Databind]
+ to your project. Then you can use `ObjectMapper` to serialize your beans:
 
 [source,java]
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