Explain the red DerivedSecrets.swift reference

The reference points into `$(DERIVED_FILE_DIR)`, so Xcode draws it red and
`Open Quickly` cannot find it — it reads as a dead reference someone should
clean up, and deleting it stops the apps compiling.

The `Derived Sources` group is repointed from `Classes` to a new `Generated`
directory to give the README somewhere to live.
Its `path` was vestigial: the only child overrides it with `sourceTree =
DERIVED_FILE_DIR`, so nothing resolved through it, and a README added under the
old path would have landed loose in the app's main source directory.

---

Generated with the help of Claude Code, https://claude.com/claude-code

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mokagio

WordPress/Generated/README.md

@@ -0,0 +1,11 @@
+# Derived Sources
+
+`DerivedSecrets.swift` is shown in red in Xcode and `Open Quickly` cannot find it.
+That is expected: it does not exist in the repository.
+
+The `Generate Credentials` build phase writes it into the building target's `$(DERIVED_FILE_DIR)` on every build, from the secrets under `~/.configure/wordpress-ios/secrets`, or, failing those, from `WordPress/Credentials/Secrets-example.swift`.
+See `Scripts/BuildPhases/GenerateCredentials.sh`.
+
+Each target gets its own copy, so `WordPress`, `Jetpack` and `Reader` cannot overwrite each other's secrets.
+
+Do not delete the red reference. The apps will not compile without it!

WordPress/WordPress.xcodeproj/project.pbxproj

@@ -523,6 +523,7 @@
 		24350E7C264DB76E009BB2B6 /* Jetpack.debug.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; path = Jetpack.debug.xcconfig; sourceTree = "<group>"; };
 		24351059264DC1E2009BB2B6 /* Jetpack.release.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; path = Jetpack.release.xcconfig; sourceTree = "<group>"; };
 		24351253264DCA08009BB2B6 /* DerivedSecrets.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DerivedSecrets.swift; sourceTree = DERIVED_FILE_DIR; };
+		24AE9EF6264B3D8E00AC7F15 /* README.md */ = {isa = PBXFileReference; lastKnownFileType = net.daringfireball.markdown; path = README.md; sourceTree = "<group>"; };
 		2439B1DB264ECBDF00239130 /* Jetpack.alpha.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; path = Jetpack.alpha.xcconfig; sourceTree = "<group>"; };
 		24AE9E66264B34E500AC7F15 /* Secrets-example.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "Secrets-example.swift"; sourceTree = "<group>"; };
 		24B54FAE2624F8430041B18E /* JetpackRelease-Alpha.entitlements */ = {isa = PBXFileReference; lastKnownFileType = text.plist.entitlements; path = "JetpackRelease-Alpha.entitlements"; sourceTree = "<group>"; };
@@ -1214,10 +1215,11 @@
 		24AE9EF5264B3D8E00AC7F15 /* Derived Sources */ = {
 			isa = PBXGroup;
 			children = (
+				24AE9EF6264B3D8E00AC7F15 /* README.md */,
 				24351253264DCA08009BB2B6 /* DerivedSecrets.swift */,
 			);
 			name = "Derived Sources";
-			path = Classes;
+			path = Generated;
 			sourceTree = "<group>";
 		};
 		29B97314FDCFA39411CA2CEA /* CustomTemplate */ = {