Addendum for `pyenv install` usage

This gives users an idea of which versions of python are available. Other binary version managers ([nvm](https://github.com/creationix/nvm) with `nvm ls-remote`, [rvm](https://rvm.io/rubies/list) with `rvm list`, [pythonbrew](https://github.com/utahta/pythonbrew) with `pythonbrew list -k`) immediately tell users how to list available versions as well.