all: document in README how x/build serves internal Go project needs
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 <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build