Update `pyenv init` line in README.md in case ```pyenv``` is not found.

* Move Homebrew to the top, common case for new devs
* Note that `brew install rbenv` includes ruby-build by default
* Be clear that `rbenv init` instructions are needed for shell setup
* Be explicit about starting a new shell to pick up PATH/init changes
* Use rbenv-doctor to conclusively demonstrate correct setup
* Separate upgrade instructions for Homebrew vs Git installs

Related: https://github.com/rbenv/ruby-build/pull/1106

I work on a team that has followed the Homebrew installations. More than once we've missed the `rbenv init` instruction and it has caused headaches down the road. This formatting makes it harder to miss.

Link was pointing to this heading. It seemed incorrect

* Updated docs to reflect homebrew change.

The instructions previously mentioned in this file were removed from the Homebrew caveats since
they weren't specific to homebrew. See discussion in [this homebrew issue](https://github.com/Homebrew/homebrew-core/pull/11209)

* Added link to specific section of readme

Document that multiple versions in `.python-version` work similarly to the command line.

Line break accomplished by two trailing spaces on the zsh lines.

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.