docs: add brew install snippet to readme

[cmoog]

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[cmoog]

We just need to make the tap public

[cmoog]

I'm considering renaming the Coder Homebrew tap such that the install command reads:

brew install cdr/tap/coder

This is similar to https://github.com/hashicorp/homebrew-tap/blob/master/Formula/terraform-ls.rb

We could also rename the formula itself to coder-cli and have an install command of

brew install cdr/coder/coder-cli
brew install cdr/tap/coder-cli

I have a feeling @kylecarbs will have strong views...