The README details `eval`ing `rbenv init -`, but for some shells (such as fish) there's a difference in what should be run. It turns out that `rbenv init` on its own will print correct instructions, so we should point users to running that command instead.

It seems rbenv now comes with ruby-build.  I have not investigated fully, but the previously shown command: `brew install rbenv ruby-build` caused issues on my machine.  After uninstalling both and simply running `brew install rbenv` everything worked fine.

[ci skip]

Closes #532

This compiles the `realpath` dynamic extension for bash which speeds up
symlink resolution. If the extension doesn't compile due to
cross-platform issues, rbenv will still work normally, although not as fast.

Removing an extra comma in "There are some systems, where the", and adding another "the" to "toward the end of shell configuration file".

Adding a tip for how to view the Homebrew package caveats again if you skipped reading them.

Directing readers to additional next steps after installing pyenv via Homebrew.

Many advanced users who enjoy reading detailed documentation may not really think of themselves as "neckbeards", even in a jokey way, so naming this section with a simple, familiar (and easy-to-translate!) name may encourage more people to read it. :)

Correcting instructions for installing Python versions (removing "download and unpack the source").

Fixing links to #pyenv-shell, #pyenv-local, and #pyenv-global - linking them to the appropriate sections of the COMMANDS.md page.

Add detail regarding activating multiple Python2 and Python3 versions simultaneously.

Closes #699, fixes #666 [ci skip]

Cfr. #688

The `rbenv versions` info highlights 1.9.3-p327 as the current version, and
making `rbenv version` sample output match that helps make it clearer.

The license information is already present in the LICENSE file.

We won't maintain it anymore. Instead, the changelogs can be obtained
from the project's Releases page: https://github.com/sstephenson/rbenv/releases

git:// protocol is blocked by a lot of servers, https works fine.

Made it consistent with the rbenv documentation by having the cloning step in the installation clone it directly into the home directory.

Add a note about how to activate/use multiple versions to the main
README, to the "Choosing the Python Version" section.

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.