Document why Jackson annotations are not used

This commit explains why Jackson annotations have no impact on the
serialization/deserialization process. Instead applications should
describe and coerce types using scalars.

Fixes gh-321

spring-graphql-docs/src/docs/asciidoc/index.adoc

@@ -256,6 +256,9 @@ You can use `RuntimeWiringConfigurer` to register:
    `AnnotatedControllerConfigurer`, which detects annotated, `DataFetcher` handler methods.
    The Spring Boot starter adds the `AnnotatedControllerConfigurer` by default.
 
+NOTE: Unlike web frameworks, GraphQL does not use Jackson annotations to drive JSON serialization/deserialization.
+Custom data types and their serialization https://www.graphql-java.com/documentation/scalars/[must be described as Scalars].
+
 The Spring Boot starter detects beans of type `RuntimeWiringConfigurer` and
 registers them in the `GraphQlSource.Builder`. That means in most cases, you'll' have
 something like the following in your configuration: