Add note that retdec can be installed via MacPorts

[catap]

The port was merged couple minutes ago and now it is available for everyone: https://ports.macports.org/search/?q=retdec&name=on

I have no idea how to make wording right and where.

[catap]

Thus, installing from mac ports requires a couple of minutes because it is prebuilded ports ;)

[xkubov]

Thank you @catap for making the RetDec available on MacPorts. I will add the note in Readme later today.