setup: Add link to MacPorts package

docs/how-to/set-up-backups.md

@@ -93,6 +93,7 @@ installing borgmatic:
  * [OpenBSD](http://ports.su/sysutils/borgmatic)
  * [openSUSE](https://software.opensuse.org/package/borgmatic)
  * [macOS (via Homebrew)](https://formulae.brew.sh/formula/borgmatic)
+ * [macOS (via MacPorts)](https://ports.macports.org/port/borgmatic/)
  * [Ansible role](https://github.com/borgbase/ansible-role-borgbackup)
  * [virtualenv](https://virtualenv.pypa.io/en/stable/)