Document the environment variables in the README

[Soviut]

I'm developing a Nuxt application inside a Docker container with a TTY connection tailing the logs. The fancy progress bars on build do not always render correctly so I wanted to disable them.

I wound up doing it the hard way by overriding the progress bar plugins in the webpack config, see here.

I only learned after the fact that the minimalCLI (which Nuxt uses to determine basic or fancy progress bars in the CLI) has environment variables that can force certain modes (eg: MINIMAL=1).

It would be really good to document the environment variables in the README.

[pi0]

I love the idea of also generally document how each detection is working

[pi0]

Added JSDocs for v3 explaining each flag.