Documentation: add "builder"-style ObjectMapper creation in README

[cowtowncoder]

With 2.10 jackson-databind has added JsonMapper.builder()... style construction for ObjectMappers. This will become the only way to construct mappers in 3.0.
It would be good to add an example in README as alternative to older direct ObjectMapper instantiation, module registration, to help users transition to the new style over time.

[k163377]

@cowtowncoder
Does the following section address this issue? (although I found a problem referencing a constructor that had already been deprecated and fixed it in #639)
https://github.com/FasterXML/jackson-module-kotlin#configuration

[cowtowncoder]

@k163377 It does, will close. Thanks!