Minor changes to make the docs better.

Closes #106

In GNU `sed`, the `-iEe` argument is equivalent to `--in-place=Ee`, which would create `~/.profileEe` as backup of `~/.profile` if the command executed successfully. However, because the `e` is no longer being processed as an expression argument, `sed` does not correctly join the expressions and exits with `sed: -e expression #2, char 10: unexpected }`.

The intent is to use extended regex, perform the changes in-place, and use a series of expressions, so `-Ei -e` is used instead.

Debian/Ubuntu's stock `~/.bashrc` prepends `~/.local/bin` to `PATH` towards its end if it exists.
Executables for per-user modules for system Python are installed into `~/.local/bin` --
so need to prepend `shims` to `PATH` later that that.

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

Change `done` to `fi` since otherwise Bash emits an unexpected token error:

```
bash: /home/foo/.bashrc: line 120: syntax error near unexpected token `done'
bash: /home/foo/.bashrc: line 120: `if command -v pyenv >/dev/null; then eval "$(pyenv init -)"; done'
```

Indirect ones with errata have proven to confuse users.
Fixes #1947, #1948

E.g. "*-latest"
May need splitting into project-specific ones if there emerge projects with different check needed

Closes https://github.com/pyenv/pyenv/issues/1559

* Update install instructions for Bash and Zsh

* Synchronize README.md with `pyenv init`
* Add a ~/.bash_profile note
* Concatenate shims activation into installation for brevity
  (Pyenv can't be used meaningfully without shims anyway)
  Otherwise, we'd need to duplicate all the ~/.profile shenanigans in both sections
* Update based on feedback
* Proofread

PATH manipulation should be done in ~/.profile rather than ~/.*rc since .rc can be sourced multiple times

Any whitespace doesn't work, need version per line.

Sometimes it is convenient to be able to temporarily disable something
in a version-file. Because these files often aren't necessarily tracked
in a SCM, especially when working with virtualenvs, the SCM diffs won't
help with showing removed lines which are currently the only way to
disable something.

The casing for `*_proxy` is currently inconsistent. I'm guessing that's a typo?

Thanks for maintaining this awesome tool!

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.