## Description
Adds the official Homebrew Cask install command for macOS to the README
installation tip.
This lets macOS users install AIRI directly with:
```sh
brew install --cask airi
```
without needing to add a custom Homebrew tap.
## Linked Issues
Related to #866
Related to #1431
## Additional Context
AIRI is now available as a Homebrew Cask, so the previous tap-based
installation discussion can be resolved with the standard cask install
flow.
Sakutaro
8b8b681a61