3.x migration guide

[krichter722]

3.x seems to introduce severe change to widely used interfaces, like removal of ObjectMapper.registerModule. It'd be nice to have a migration guide which offers an overview of such popular 2.x use cases, recipes and idioms and their 3.x counterpart in case they changed.

I'm aware that 3.x is not released, yet, so this is merely a reminder.

[cowtowncoder]

Yes, absolutely.

More discussions will also be needed since I think my original idea of 2.x -> 3.x being smaller step than 1.x -> 2.x is proving incorrect.

Biggest change for ObjectMapper and JsonFactory will be change to fully immutable implementation, which makes use of Builder-pattern necessary. One (and about only) backwards-compatibility feature is that zero-argument constructors are left in place, to create "vanilla" instances.