corrected fish shell command

This prevents people from jumping the gun and trying to install a python version right after brew install.

Borrowed from rbenv.

This makes it easier to copy and paste commands from the README.

Bash example had a missing quote for setting PYENV_ROOT path

Describe more clearly for different shells.

Update the bats link to https://github.com/bats-core/bats-core (which is what pyenv is actually using on [Travis](https://github.com/pyenv/pyenv/blob/master/.travis.yml#L35))

I skimmed past the requires post-installation steps for Homebrew when walking though the README. 
Anecdotally, other devs noted the same issue.

https://github.com/pyenv/pyenv/issues/1400#issuecomment-551371058

Closes #846, #1400, #1447, #1449

Closes #1485

see #62

Adding a list to the notes of point #2 in Basic GitHub Checkout steps.

Replace OS X with macOS

This also fixes the aspect ratio of the image being incorrect.

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.