REQUEST: add more details to readme.md

[asampal]

It would be great and be much appreciated if, for SBT newbies like me, readme.md could clearly spell out the differences between the SBT functionality offered by this plugin where it is extra, compared to what is available through the Scala plugin, and where it overlaps (if it does, in fact).

[unoexperto]

@asampal sbt-idea is redundant because new SBT plugin for Intellij IDEA can open SBT projects.

[asampal]

Well, in that case (and if there's no difference in the importing ability) it would make sense to state this redundancy in the readme. It's not obvious to someone not familiar with SBT and the ecosystem around it.