Update README.md for current state of macOS
Update README.md for current state of macOS

Homebrew Rust will correctly compile `alacritty` now, and the monospace
font is automatically set to `Menlo` at first launch.

alacritty/alacritty,alacritty/alacritty,jwilm/alacritty,jwilm/alacritty,jwilm/alacritty,jwilm/alacritty