Update Usage.md (#2369)
Update Usage.md (#2369)

* Update Usage.md

I might have this totally wrong but this bit confused me. Especially since later in the README it explicitly states:

> SwiftPM allows two build configurations: Debug (default) and Release.

If what I've added is not the right way to obtain / use the `SWIFT_PACKAGE` flag, please let me know. 

Thanks!

* Update Usage.md

apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager