nvm: add README (#7202)

plugins/nvm/README.md

+# nvm plugin
+
+This plugin adds autocompletions for [nvm](https://github.com/creationix/nvm) — a Node.js version manager.
+It also automatically sources nvm, so you don't need to do it manually in your `.zshrc`.
+
+To use it, add `nvm` to the plugins array of your zshrc file:
+```zsh
+plugins=(... nvm)
+```