Add README section about zsh-nvm's AUTO_USE option

[gimbo]

Add a subsection to README.md about using zsh-nvm's NVM_AUTO_USE option to automatically detect and use .nvmrc files; the zsh-nvm plugin is mentioned already in the README (in the installation section).

[ljharb]

I'm not sure it makes sense to document "not nvm" here; presumably this is in zsh-nvm's own docs?

[gimbo]

🤷‍♂️ Perhaps. There's quite a lot of space in the README devoted to automatically detecting/running .nvmrc files, with script examples, and which also specifically mentions avn as helping with this too. zsh-nvm is already mentioned earlier in the README, so to me it's just mentioning an alternative to avn for us zsh users. Compared to the amount of space devoted to explaining how to do it by hand this is a short section.

I don't have any skin in this game though, I was just glad that I spotted that zsh-nvm (which I was already using) could do this before I spent any time installing and configuring avn, and I thought other people might benefit from that too.

[ljharb]

It'd be fine to link to the zsh-nvm docs, but we should avoid any prose in nvm's readme that other maintainers could invalidate.