Update instructions to include node setup for Apple Silicon laptops

I have a new M1 Mac and following the steps in https://github.com/fastruby/setup/blob/main/mac.sh I wasn't able to get node to work. I've used yarn in the past so I figured I was doing something wrong with nvm, but it turns out you need the latest version of node (16) for Apple Silicon support, and the script is using an older version. See here: https://www.infoworld.com/article/3615673/nodejs-16-introduces-apple-silicon-support.html

Specifically, when I updated my .zshrc file, this happened:
```
~ $ source .zshrc
Please see `nvm --help` or https://github.com/nvm-sh/nvm#nvmrc for more information.
```

So there was an error, but I couldn't tell what it was (the output from the error wasn't being shown for some reason). I tried a variety of things, but eventually figured it out with:

```
~ $ node -v
zsh: bad CPU type in executable: node
```

Also, what I ultimately put in my .zshrc file is different from what's in the mac.sh script. I used what was recommended by homebrew after installing nvm:

```
Add the following to ~/.zshrc or your desired shell
configuration file:

export NVM_DIR="$HOME/.nvm"
[ -s "/opt/homebrew/opt/nvm/nvm.sh" ] && \. "/opt/homebrew/opt/nvm/nvm.sh" # This loads nvm
[ -s "/opt/homebrew/opt/nvm/etc/bash_completion.d/nvm" ] && \. "/opt/homebrew/opt/nvm/etc/bash_completion.d/nvm" # This loads nvm bash_completion
```