update Readme for Mojave

[justvanbloom]

Hi,

for macOS Mojave

hdiutil create -o /tmp/mojave.cdr -size 7316m -layout SPUD -fs HFS+J hdiutil attach /tmp/mojave.cdr.dmg -noverify -mountpoint /Volumes/install_build sudo /Applications/Install\ macOS\ 10.14\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/install_build mv /tmp/mojave.cdr.dmg ~/Desktop/InstallSystem.dmg hdiutil detach /Volumes/Install\ macOS\ 10.14\ Beta hdiutil convert ~/Desktop/InstallSystem.dmg -format UDTO -o ~/Desktop/mojave.iso