example doc for enum and case objects

[davideicardi]

I have tried to add some documentation example on how to implement json formats for enumeration and case object types.

I think that both are quite common in Scala and providing an official example can be useful.

[ndlarsen]

This was actually quite helpful to me. Thank you

[waagnermann]

And on PR itself: why it's still not merged if @raboof and @ndlarsen say it's helpful?