Add "Creating a Macro Package" to Usage.md

[BrianHenryIE]

Add a section containing and describing swift package init --type macro.

Motivation:

I was looking for the documentation and it wasn't there.

Modifications:

Added the section by copying the Library Package and Executable Package sections' format, showing the command line input and output, describing the output of InitPackage.swift and linking to documentation for the sample macro.

Result:

Only one file is changed, which is documentation, so no change in behaviour of Swift Package Manager.

[neonichu]

Thanks, this looks good!

Could you also add a link to https://docs.swift.org/swift-book/documentation/the-swift-programming-language/macros in there?

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test

[neonichu]

Looks like we're failing to build the compiler on the Linux job (obviously unrelated to this PR):

/home/build-user/swift/SwiftCompilerSources/Sources/Basic/SourceLoc.swift:73:49: error: value of type 'swift.CharSourceRange' has no member 'getByteLength'
    self.init(start: start, byteLength: bridged.getByteLength())
                                        ~~~~~~~ ^~~~~~~~~~~~~

[MaxDesiatov]

Thanks!

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows