feat: Initial arr CLI.md + setup instructions

[charlesvien]

Trying out the arr cli for an extended period @jonathanlab 💙

Wanted to add a little bit of setup instructions for anyone else wanting to try it out, could be a bit too early, entirely up to you!

[jonathanlab]

hi @charlesvien, thanks for this! Totally, go ahead. Just be mindful of #443 and #470 if you run into any issues. We also already have a README.md for the arr CLI. How about we pull all instructions for Array and arr in a /docs folder? Think that would be most maintainable, especially if we'll use those as a source of truth for the website later on

[charlesvien]

hi @charlesvien, thanks for this! Totally, go ahead. Just be mindful of #443 and #470 if you run into any issues. We also already have a README.md for the arr CLI. How about we pull all instructions for Array and arr in a /docs folder? Think that would be most maintainable, especially if we'll use those as a source of truth for the website later on

I'm silly, I updated the PR to just make the main README aware of the CLI