Add documentation about XQuartz to setup page

[marwahaha]

Cool project 👍

I got the install working via ./setup.sh on my Mac running Sierra 10.12.6. I did not have Geant or ROOT before this install.

Only one hiccup:
There was an issue when the cmake command installing Geant was looking for X11, and my machine didn't have have it. Looks like Apple doesn't install it by default anymore -- https://support.apple.com/en-us/HT201341 , but it's a simple install! https://www.xquartz.org/

After installing, the ./setup.sh script worked great!

[zoglauer]

Hello,

Thanks!
The xquartz information is already included in the extended setup manual, which is linked from the main README.md:
http://megalibtoolkit.com/setup.html
Section "Additional information for macOS"

Best,
Andreas

[marwahaha]

Thanks, I missed that!