Add "Creating a Macro Package" to Usage.md (#6944)

* Add "Creating a Macro Package" to `Usage.md`
* Fix documentation link
* Add link to The Swift Programming Language book

Co-authored-by: Max Desiatov <[email protected]>

Author: BrianHenryIE

Documentation/Usage.md

@@ -7,6 +7,7 @@
   * [Creating a Package](#creating-a-package)
     * [Creating a Library Package](#creating-a-library-package)
     * [Creating an Executable Package](#creating-an-executable-package)
+    * [Creating a Macro Package](#creating-a-macro-package)
   * [Defining Dependencies](#defining-dependencies)
   * [Publishing a Package](#publishing-a-package)
   * [Requiring System Libraries](#requiring-system-libraries)
@@ -68,6 +69,25 @@ can be turned into a executable target if there is a `main.swift` file present i
 its sources. The complete reference for layout is
 [here](PackageDescription.md#target).
 
+### Creating a Macro Package
+
+SwiftPM can generate boilerplate for custom macros:
+
+    $ mkdir MyMacro
+    $ cd MyMacro
+    $ swift package init --type macro
+    $ swift build
+    $ swift run
+    The value 42 was produced by the code "a + b"
+
+This creates a package with a `.macro` type target with its required dependencies
+on [swift-syntax](https://github.com/apple/swift-syntax), a library `.target` 
+containing the macro's code, and an `.executableTarget` and `.testTarget` for 
+running the macro. The sample macro, `StringifyMacro`, is documented in the Swift 
+Evolution proposal for [Expression Macros](https://github.com/apple/swift-evolution/blob/main/proposals/0382-expression-macros.md)
+and the WWDC [Write Swift macros](https://developer.apple.com/videos/play/wwdc2023/10166) 
+video. See further documentation on macros in [The Swift Programming Language](https://docs.swift.org/swift-book/documentation/the-swift-programming-language/macros/) book.
+
 ## Defining Dependencies
 
 To depend on a package, define the dependency and the version in the manifest of