Added example of how to apply json custom converter in freezed documentation

[SandroMaglione]

The PR aims to clarify two features from #240 and #232.

It adds two code examples on how to apply the custom converter decorator MyResponseConverter already present inside the documentation. Specifically, one example illustrates how to apply the decorator by passing it as a constructor parameter, while the second one shows that the converter works also on a List parameter.

It also adds a note below it to clarify that, in order to serialize nested list of freezer objects, you are supposed to either specify a @JsonSerializable(explicitToJson: true) or change explicit_to_json inside your build.yaml file (taken from here).

Note: The PR introduces a repetition. The reference to build.yaml to define some custom json_serializable flags is present both inside the PR and at the very bottom of the original documentation. I think the note may be helpful right below the json_serializable section to clarify the concepts explained above.

[rrousselGit]

Thanks!