Add documentation block for using homebrew

[mheob]

Add a documentation block for installing the font via homebrew on a MacOS.

Addition to and closes #14

[altaywtf]

shouldn't we do brew tap homebrew/cask-fonts first?

brew tap homebrew/cask-fonts
brew install font-monaspace

worked out for me, but brew install --cask font-monaspace didn't.

[mheob]

You are right. Since I had already tapped homebrew/cask-fonts, I forgot that we have to call it at least once.

I added it now.

[krehel]

Thanks for adding the instruction for Homebrew!

[idan]

Thank you!!