Document rbenv::client

[fgrehm]

I've just ran into this guy today and haven't seen anything about it on the README, I'll give a shot at using it later on and contribute back some documentation if you dont have the time to do it.

[alup]

@fgrehm this would be perfect :)!

[andyleejordan]

Yeah, on this note, the rbenv::bundle requires an rbenv::client resource for the user, but just what does rbenv::client do?

I use rbenv::install to actually install rbenv, rbenv::compile to add my needed ruby version, and I want to use rbenv::bundle to install dependencies in a particular project folder. Where does rbenv::client come into play?