Add changelog entry for manifest dummy targets

Author: aciidgh

CHANGELOG.md

@@ -2,6 +2,11 @@ Note: This is in reverse chronological order, so newer entries are added to the
 
 Swift 4.0
 ---------
+
+* The generated Xcode project creates a dummy target which provides
+  autocompletion for the manifest files. The name of the dummy target is in
+  format: `<PackageName>PackageDescription`.
+
 * `--specifier` option for `swift test` is now deprecated.
   Use `--filter` instead which supports regex.