README: add Macports install instructions

[herbygillot]

No description provided.

[sassman]

Nice.

I'm not so familiar with macports, could you tell me who is maintaining the macports version?
And how it is added, or what's the process of keep things updated on new releases?

For homebrew there is a Auto PR as part of the build pipeline. I wonder if macports needs the same?

[herbygillot]

So the way MacPorts works is that every piece of software is a "port", and has an associated Portfile.

Here's the current one for t-rec: https://github.com/macports/macports-ports/blob/master/graphics/t-rec/Portfile

I'm the current maintainer for t-rec's port, though anyone can submit a PR to update ("openmaintainer" in the maintainers section).

Unfortunately MacPorts doesn't have an auto-PR process just yet, so as of right now, it relies on maintainers to submit changes on version update.

Portfiles look different depending on the programming language, packaging or module system used for the software in question, but t-rec's portfile is pretty consistent for a port based on cargo. On update, the version no. in "github.setup", the checksums, and the relevant crates all see an update (you can see examples of this browsing the Portfile's history).