Addendum for `pyenv install` usage

This gives users an idea of which versions of python are available. Other version managers ([nvm](https://github.com/creationix/nvm), [rvm](https://rvm.io/rubies/list), etc) tell users how to list available versions as well.
Merge pull request #125 from shurane/patch-1

Addendum for `pyenv install` usage