README.md: add info about MacPorts port into the list

[barracuda156]

Non-functional change: add info about MacPorts port with a link to it.

[sp1ritCS]

Some notes:

• What you've packaged as "MicroTeX" is actually cLaTeXMath. You should probably rename that package (or package the openmath branch of that repo, but you'd need to apply [notekit#e50072a](https://github.com/sp1ritCS/notekit/commit/e50072a491e607e1b7acb8c045534b4443f14040.patch which still needs testing)
• For homebrew I did the questionable decision to install Apple/NoteKit.app and put a symlink to the binary in there, that a user can copy that directory into their "Applications" to be able to launch notekit from their desktop (and have an app icon). Maybe you want to do something similar

Also I believe gtk-mac-bundler can work with MacPort. If you have the time for it, having an GHA run that builds notekit using MacPort and bundles it into a single "application" would be appreciated.