Add documentation for default plugins installed

[ketozhang]

A few plugins are installed by default:

https://github.com/pyenv/pyenv-installer/blob/49fba599e872bc761858ea6f700271fb6dcb5a97/bin/pyenv-installer#L46-L51

Please,

• Add to README, a list of the default plugin installed by the installer
• Add an option to disable installing these plugin, or don't install any plugins by default.

[izzatkarimov]

Hello, is the issue solved? Or can I go ahead and solve it?

[ketozhang]

No, go ahead.