Minor additions to Scala module README, strong encourage PRs for that, others

[Tatu Saloranta]

So: with addition of PJ as a new core developer for Scala module, I
realized that it'd make sense to include at least basic list of core
developers on README:

https://github.com/FasterXML/jackson-module-scala

since while info is of course available with Team/collaborators page
under Settings, that's not as obvious as README, nor can include other
information on roles, responsibilities, availability and so on.
Ideally over time README would get more details like that, but let's
start with basics.

With that, it would be absolutely GREAT to get PRs (or just issues
filed, for minor changes) for READMEs of various projects.

I specifically hope to improve Kotlin module page, as we get new core
developers.

-+ Tatu +-