OS X ships with editline rather than readline, and by default any Ruby you compile on it will use the former rather than the latter. This rbenv plugin's sole purpose is to tell ruby-build to link to a Homebrew installed readline instead.
The particular problem this was created to solve was getting the ironically named pry-editline to consistently work across my Ruby installs. This is by no means the only reason to eschew editline.
git clone git://github.com/tpope/rbenv-readline.git \
~/.rbenv/plugins/rbenv-readline