docs(nvm): clarify how to enable settings (#9542)

It wasn’t clear where I should set the `NVM_AUTOLOAD` variable. The clue was to be found in [the docs for another plugin](https://github.com/ohmyzsh/ohmyzsh/tree/master/plugins/fzf). It seems setting values need to come before Oh My Zsh is sourced.

Updated read me to reflect this, and the fact that one of the settings mentioned is specific to nvm, not this plugin.