Update README: highlight brew post-install steps (#1543)

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

README.md

@@ -299,6 +299,8 @@ package manager for macOS.
 
 To upgrade pyenv in the future, use `upgrade` instead of `install`.
 
+#### Post-installation
+
 Then follow the rest of the post-installation steps under [Basic GitHub Checkout](https://github.com/pyenv/pyenv#basic-github-checkout) above, starting with #3 ("Add `pyenv init` to your shell to enable shims and autocompletion").
 
 ### Advanced Configuration