Migrate readme to v3

[snapsl]

The readme currently contains many examples that are not compatible with the v3 release.

In many cases the "abstract" is missing or the old factory syntax mixes with the new class syntax.

[lume-code]

It appears that freezed does not generate fromJson/toJson using the previous syntax anymore. That and not knowing the course moving forward prevented me from upgrading to v3. Adding @JsonSerializable annotation didn't help either per the migration_guide.md documentation.

[snapsl]

@lume-code Use the abstract or sealed keyword respectively for factory constructors as mentioned at the beginning of the migration guide. The @JsonSerializable annotation is used for the new classic class syntax.

[lume-code]

@snapsl thanks, It was already sealed but same results.

[snapsl]

Could be a bug. Please fill a new issue containing your freezed class.