Adding R Setup instructions for Windows and OS X

Hi,
I recently checked out the h2o-dev project, followed the setup instructions in the README.md for both Win 7 64 and OS X, and to my dismay the builds failed. At that point I had to debug the build and google around to find the missing packages.

It would have saved me a bunch of time if the links/information that I'm including with this patch had been there for me when I started out.