Add instructions for installing Xcode (#125)

Now, we require installing Xcode 26 beta. This PRs adds an important
step of setting the active developer directory. Fixes
https://github.com/apple/containerization/issues/119.

README.md

@@ -71,6 +71,13 @@ A kernel image named `vmlinux.container` can be found in the `/opt/kata/share/ka
 
 ## Prepare to build package
 
+Install the recommended version of Xcode.
+
+Set the active developer directory to the installed Xcode (replace `<PATH_TO_XCODE>`):
+```bash
+sudo xcode-select -s <PATH_TO_XCODE>
+``` 
+
 Install [Swiftly](https://github.com/swiftlang/swiftly), [Swift](https://www.swift.org), and [Static Linux SDK](https://www.swift.org/documentation/articles/static-linux-getting-started.html):
 
 ```bash

vminitd/Makefile

@@ -69,14 +69,15 @@ linux-sdk:
 
 .PHONY: macos-sdk
 macos-sdk:
+	# Consider switching back to `xcode-cltools`, when possible.
 	@if [ $(MACOS_MAJOR) -gt 15 ] && [ "$(MACOS_RELEASE_TYPE)" = "" ]; then \
-	  "$(MAKE)" xcode-cli; \
+	  "$(MAKE)" xcode; \
 	else \
 	  "$(MAKE)" xcode; \
 	fi
 
-.PHONY: xcode-cli
-xcode-cli:
+.PHONY: xcode-cltools
+xcode-cltools:
 	@echo Activating Xcode Command Line Tools...
 	@sudo xcode-select --switch /Library/Developer/CommandLineTools