Explain what homebrew is

[tiagoboldt]

Clarify what homebrew is in the readme.

[0xdevalias]

Whoops.. seems like there are already a few duplicate PRs to fix this:

• Update brew installation to the the latest format #2003
• use brew install --cask instead of brew cask #2148

And not strictly a duplicate, but another update to the homebrew section of the docs:

• Explain what homebrew is #1660

Originally posted by @0xdevalias in #2149 (comment)

[tiagoboldt]

To be fair, this one's been open since 2019 without any review, so I expect it to be somehow deprecated. I'm happy to update it to what makes sense today or for it to be closed in favor of something more recent.