docs: add Discord community badge to README

[ulises-jeremias]

Context

Align packaging README with other CPA repos: Discord badge → https://discord.gg/bR5VyATgka

[ulises-jeremias]

Taking this one myself — Homebrew/AUR packaging work needs org-specific permissions (tap publish, AUR credentials, distribution secrets), so it is not a good fit for external contributors right now.

If you want to help with related docs/tests that do not require those secrets, ping me in Discord and we can carve out a safe slice.