Update MacPorts links in README

[contextnerror]

Expected Behavior

The MacPorts package should no longer be listed as pending, and the link to the port should be current. This would be either https://ports.macports.org/port/markdown-mode.el/ or https://github.com/macports/macports-ports/blob/master/editors/markdown-mode.el/Portfile depending on if a link directly to the source is desired.

Actual Behavior

The port is currently described as pending. The link also leads to trac, which is several years behind the latest change.

Steps to Reproduce

N/A

Backtrace

N/A

Software Versions

N/A