all: document in README how x/build serves internal Go project needs

Help users avoid incorrect expectations of stability, and
help us avoid incurring needless maintenance overhead.

For golang/go#51867.
Fixes golang/go#36505.

Change-Id: I78377711ff2d5441bcdc4186dc8708167cf1a952
Reviewed-on: https://go-review.googlesource.com/c/build/+/394514
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

README.md

@@ -2,9 +2,15 @@
 
 [![Go Reference](https://pkg.go.dev/badge/golang.org/x/build.svg)](https://pkg.go.dev/golang.org/x/build)
 
-This subrepository holds the source for various packages and tools that support
+This repository holds the source for various packages and tools that support
 Go's build system and the development of the Go programming language.
 
+**Warning:** Packages here are internal to Go's build system and its needs.
+Some may one day be promoted to another `golang.org/x` repository,
+or they may be modified arbitrarily or even disappear altogether.
+In short, code in this repository is not subject to the Go 1 compatibility
+promise nor the [Release Policy](https://go.dev/doc/devel/release#policy).
+
 ## Report Issues / Send Patches
 
 This repository uses Gerrit for code changes. To contribute, see