More detail for RVM.md

[nanoamp]

I haven't used RVM for ages; it normally just sits there quietly overriding the OS X default ruby install for me. So, when I hit these instructions, it took me a few minutes to re-orient myself. I've added some more detail to explain how to retrieve the info you need, and replaced the absolute path reference (which isn't correct for homebrew installations of rvm). Hope this may be useful for others.