README: Fixed typo, deadlink, and added macOS path options for MacPorts Users

[samyarvahid]

Hi, Thanks for making this program, I have some small additions to the README.md that could be worthwhile:

• fixed typo: 'executible' to 'executable'
• fixed outbound link to go-mpv repo
• changed OS X to macOS
  • in anticipation of potential detailed mac build notes, more on that below
• added path setup for MacPorts users, as per @dweymouth's mac setup instructions for supersonic, which have been mentioned a few times in some replies to various issues in this repo. The Homebrew path setup has since been included, but not the MacPorts one.

I didn't include it in this branch, but I made a tentative second branch that I could make another PR for, where I describe what needs to be done to get stmp working on macOS versions < 10.15, by building a version older than 0.37, or by using the macports mpv-legacy package.

Finally, I don't know if this is too out-of-scope, but as someone unfamiliar with go, it took me a little bit to realize that I needed to go get the go build dependencies. I could make that more clear with another couple lines in the readme, if it's of interest.

Thanks!