SwiftExtract: document the dummy.json placeholder in Package.swift

The `Resources/dummy.json` placeholder exists only so SwiftPM emits a
`Bundle.module` for the SwiftExtract target — the real
`static-build-config.json` is generated at build time by
`_StaticBuildConfigPlugin`. Note that on the `.process("Resources")`
line so a future reader doesn't try to delete the empty-looking file.

Author: ktoso

Package.swift

@@ -354,6 +354,9 @@ let package = Package(
       ],
       path: "Sources/SwiftExtract",
       resources: [
+        // Holds the `dummy.json` placeholder so SwiftPM emits a `Bundle.module`
+        // for this target. The real `static-build-config.json` is generated at
+        // build time by the `_StaticBuildConfigPlugin` build tool below.
         .process("Resources")
       ],
       swiftSettings: [