[docs] GettingStarted.md: Recommend building the `--release` variant of `--xcode`
[docs] GettingStarted.md: Recommend building the `--release` variant of `--xcode`

The `--release` variant takes up 4 times less space than the default `--debug`
variant (0.45GB vs 1.8GB).

apple/swift,apple/swift,apple/swift,apple/swift,apple/swift,apple/swift