markdown-mode.el

• If not done already, install MacPorts.
• To install markdown-mode.el, run the following command in macOS terminal (Applications->Utilities->Terminal)
  
  sudo port install markdown-mode.el Copy
• To see what files were installed by markdown-mode.el, run:
  
  port contents markdown-mode.el Copy
• To later upgrade markdown-mode.el, run:
  
  sudo port selfupdate && sudo port upgrade markdown-mode.el Copy

The MacPorts Project uses a system called Trac to file tickets to report bugs and enhancement requests. Though anyone may search Trac for tickets, you must have a GitHub account in order to login to Trac to create tickets.