Replacing setx in the readme.md

[LillyPK]

Hello, I noticed that the readme file recommends using setx but setx is old and it can only handle 1024 characters in the path variable. If you have more than 1024 characters in your path variable and try to use setx, your environment variable will get corrupt.
Since it mentions shell I'm assuming we're already using power shell or windows terminal from the beginning so I recommend something like this command instead of setx

[Environment]::SetEnvironmentVariable("Path", $env:Path +";C/folder directory", "Machine")

I know setx is easier to use but it's bit me more times than I can count

[dunhor]

While we'd welcome a PR to update it (ideally to mention both, not replace), this doesn't sound likely to be a big issue since the suggestion is a new environment variable (VCPKG_ROOT), not updating on existing one (like PATH). Most paths will be less than MAX_PATH and even more will be less than 1024 characters in length.