contributing update

CONTRIBUTING.md

@@ -1225,7 +1225,13 @@ $ yarn test
 ## Changing Cloud Assembly Schema
 
 If you plan on making changes to the `cloud-assembly-schema` package, make sure you familiarize yourself with
-its own [contribution guide](./packages/@aws-cdk/cloud-assembly-schema/CONTRIBUTING.md)
+its own [contribution guide](https://github.com/cdklabs/cloud-assembly-schema/blob/main/CONTRIBUTING.md).
+
+The team will not actively monitor the `cloud-assembly-schema` repository for contributions. If you have a PR
+that needs to modify `cloud-assembly-schema`, open both PRs and ensure that you link the `cloud-assembly-schema`
+PR in the one opened against this repository. The `cloud-assembly-schema` changes must be merged (and released)
+before the PR in this repository can be merged. The PR in this repository must be updated to use the new release
+of the `cloud-assembly-schema` before it can be merged.
 
 ## Feature Flags