suggest running setup.sh as root for App Store-installed Xcode

[mykmelez]

The App Store installs /Applications/Xcode.app as root/wheel:

> ll -d /Applications/Xcode.app

drwxr-xr-x@ 3 root wheel 96B May 17 07:29 /Applications/Xcode.app

> ll -d /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageSpecifications

drwxr-xr-x 52 root wheel 1.6K May 20 09:10 /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageSpecifications

> ll -d /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageMetadata

drwxr-xr-x 58 root wheel 1.8K May 20 09:10 /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageMetadata

Which prevents copying the language specification and metadata files when running setup.sh as a regular user:

> ./setup.sh
…
+ cp Specifications/Rust.xclangspec /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageSpecifications
cp: /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageSpecifications/Rust.xclangspec: Permission denied
…
+ cp Xcode.SourceCodeLanguage.Rust.plist /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageMetadata
cp: /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageMetadata/Xcode.SourceCodeLanguage.Rust.plist: Permission denied

This branch adds a note to the readme that you may have to run setup.sh as root. It also suggests that solution in the output of setup.sh itself if the cp commands fail.