update readme for node 22 update

[oohwooh]

I think it's better to link directly to the nvm install instructions rather than copy pasting them in case the steps ever change (and so people don't install an outdated version)

I also created a .nvmrc file so you don't need to pass any arguments to nvm install or nvm use - see docs here

[vercel]

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[oohwooh]

@HanCotterell do you think I simplified it too much? I don't want people getting confused. I am happy to go back to the original one and just update it to remove the arg from the nvm install and nvm use commands.

[oohwooh]

Maybe we should also add something saying to use WSL if on Windows, and if there's anything different people need to do on a Mac