Add installation instructions for macOS

[joschi]

# brew info sengi
sengi: 1.0.3
https://github.com/NicolasConstant/sengi
Not installed
From: https://github.com/Homebrew/homebrew-cask/blob/HEAD/Casks/sengi.rb
==> Name
Sengi
==> Description
Mastodon and Pleroma desktop client
==> Artifacts
Sengi.app (App)
==> Analytics
install: 0 (30 days), 0 (90 days), 0 (365 days)

Refs https://github.com/Homebrew/homebrew-cask/pull/98773