Keychain package

* Added the Keychain package
* Added a Makefile to easily build the package
* Updated the README with additional information

Makefile

@@ -0,0 +1,13 @@
+.PHONY: build resolve clean
+
+build:
+	@echo "Building Swift package 'Keychain'"
+	@swift build --package-path Keychain --scratch-path .build
+
+resolve:
+	@echo "Resolving Swift package dependencies..."
+	@swift package resolve --package-path Keychain --scratch-path .build
+
+clean:
+	@echo "Cleaning build artifacts..."
+	@rm -rf .build Keychain/.swiftpm