Include cargo_build_script in the policy

COMPATIBILITY.md

@@ -72,9 +72,11 @@ that anything directly accessible from this file is considered stable.
 `//rust/private/…` is not subject to the backwards compatibility policy. Content
 of this package is an implementation detail.
 
-`//cargo`, `//util`, `//tools`, `//test`, `//examples`, `//bindgen`, `//proto`, `//wasm_bindgen`
-and any packages not mentioned by this document are by default not subject to
-the backwards compatibility policy.
+`//cargo:cargo_build_script.bzl` is subject to the backwards compatibility policy.
+
+`//cargo`, `//util`, `//tools`, `//test`, `//examples`, `//bindgen`, `//proto`,
+`//wasm_bindgen` and any packages not mentioned by this document are by default
+not subject to the backwards compatibility policy.
 
 Experimental build settings are not subject to the backward compatibility
 policy. They should be added to `//rust:experimental.bzl`.
@@ -85,7 +87,9 @@ They should be added to `//rust:incompatible.bzl`.
 
 Bug fixes are not a breaking change by default. We'll use Common Sense (and we
 will pull in more maintainers and the community to discuss) if we see a certain
-bug fix is controversial.
+bug fix is controversial. Incompatible changes to
+`//cargo:cargo_build_script.bzl` that make `cargo_build_script` more accurately
+follow cargo's behaviour are considered bug fixes.
 
 ## How to make a backwards incompatible change?