Update documentation to reflect (anticipated) v0.5.0

First draft for documentation update. A separate SBT-plugin page will follow.
This paragraph can be restated, saying that Scala primitive types should be used instead of `js.prim._`, with possibly using `scala.Int` where applicable (stress that `Int` is to be used only if the value is in the range of an `Int`, 32-bits; if an integer value may be out of that range -which is typically the case for time values-, `Double` must be used -and not `Long`).