[arch-general] Instructions to mount efivars as readonly should be linked to in Beginner's Guide

Hi, I have read the dangers of mounting efivars as writable recently, and I think there should be an entry in the archlinux installation guide and beginner's guide which should say exactly what is said in the warning in [1], and then link to [1] for further instructions. -- Cheers Jayesh Badwaik [1] https://wiki.archlinux.org/index.php/ Unified_Extensible_Firmware_Interface#Mount_efivarfs