Update Readme.MD following changes from 0.3.0

Since version 0.3.0, setting the following property doesn't seem to be sufficient anymore : spring.http.converters.preferred-json-mapper: json-api (As it was also done in your example project here : https://github.com/oliverlj/json-api-converter-spring-boot-example)
Could you please mention in the "readme.md" file the fact that from version 0.3.0, it is also necessary to add the following code in the application class to enable the json-api serialization / deserialization feature ?

  @Bean
  public ObjectMapper objectMapper() {
    return new ObjectMapper();
  }

I happened to find this by looking at your commits but it was not so easy to find.
By the way, thank you for your great job !