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.

plugins/nvm/README.md

@@ -11,9 +11,11 @@ plugins=(... nvm)
 
 ## Settings
 
-- **`NVM_DIR`**: if you have installed nvm in a directory other than `$HOME/.nvm`, set and export `NVM_DIR`
-  to be the directory where you installed nvm.
-  
+If you installed nvm in a directory other than `$HOME/.nvm`, set and export `NVM_DIR` to be the directory
+where you installed nvm.
+
+These settings should go in your zshrc file, before Oh My Zsh is sourced:
+
 - **`NVM_HOMEBREW`**: if you installed nvm via Homebrew, in a directory other than `/usr/local/opt/nvm`, you
   can set `NVM_HOMEBREW` to be the directory where you installed it.