Document opt-in auto-use recipe in README

[esimkowitz]

Summary

Adds a README section showing how users can opt into automatic nvm use on cd, mirroring the recipe-style approach used in the upstream nvm-sh README.

The snippet:

• Registers a --on-variable PWD handler so it only runs in interactive shells
• Walks up from $PWD to find the nearest .nvmrc or .node-version
• Calls nvm use --silent when a file is found
• Caches the last activated file so it doesn't re-fire when you cd between subdirectories of the same project
• Runs once at config load so freshly opened shells already have the right version

Alternative to #252

This is a documentation-only alternative to #252, which added a built-in nvm_auto_use variable. Keeping the behavior as a README recipe avoids growing the plugin's surface area and lets users tweak the function for their own preferences (e.g., falling back to a default version, integrating with other tools). I'll close #252 in favor of this approach.

Test plan

• Sourced the snippet in an interactive fish shell and verified that cd-ing into a directory with a .nvmrc fires the hook
• Verified subsequent cds within the same project are no-ops (cached file path)
• Verified the hook does not fire in non-interactive shells (fish -c '...')

[jorgebucaran]

This function is a bit too niche. We should provide a minimal boilerplate so people can start experimenting.

Also, this seems to bypass nvm use entirely.

nvm.fish/functions/nvm.fish

Lines 196 to 201 in cc70373

	function _nvm_find_up --argument-names path file
	test -e "$path/$file" && echo $path/$file || begin
	test ! -z "$path" || return
	_nvm_find_up (string replace --regex -- '/[^/]*$' "" $path) $file
	end
	end