Set R modular to stable in README.md. Update Interfaces doc and NEWS.

I guess that the R modular bindings could be considered "stable" for now since they run all of the existing examples. It'll be a bit more work before they're as good as the Python bindings though. I updated the Interfaces doc page too, and while I was at it I moved the modular interfaces to the top of the page since (as far as I know) they're the preferred way to use interface with Shogun.