Add a disclaimer to readme

apple-llvm-split-commit: e2780815a0b31c54a49377fbb50359898e87b25e
apple-llvm-split-dir: -

Author: hyp

README.md

@@ -1,3 +1,14 @@
+# Disclaimer
+
+The [llvm-monorepo-root](https://github.com/apple/llvm-monorepo-root),
+the [llvm-project-v1](https://github.com/apple/llvm-project-v1), and
+the [llvm-project-v1-split](https://github.com/apple/llvm-project-v1-split) are
+WIP repositories that are used for development and prototyping of an llvm-project monorepo
+for Swift. Please see [this forum post](https://forums.swift.org/t/llvm-monorepo-transition/25689)
+for more details.
+
+Original readme follows:
+
 # The LLVM Compiler Infrastructure
 
 This directory and its subdirectories contain source code for LLVM,