Clarify bundled package release process in Fleet README (#129373)

x-pack/plugins/fleet/README.md

@@ -163,4 +163,5 @@ Fleet supports shipping integrations as `.zip` archives with Kibana's source cod
 
 The set of bundled packages included with Kibana is dictated by a top-level `fleet_packages.json` file in the Kibana repo. This file includes a list of packages with a pinned version that Kibana will consider bundled. When the Kibana distributable is built, a [build task](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/bundle_fleet_packages.ts) will resolve these packages from the Elastic Package Registry, download the appropriate version as a `.zip` archive, and place it in a directory configurable by a `xpack.fleet.bundledPackageLocation` value in `kibana.yml`. By default, these archives are stored in `x-pack/plugins/fleet/.target/bundled_packages/`. In CI/CD, we [override](https://github.com/elastic/kibana/blob/main/x-pack/test/fleet_api_integration/config.ts#L20) this default with `/tmp/fleet_bundled_packages`.
 
-Until further automation is added, this `fleet_packages.json` file should be updated as part of the release process to ensure the latest compatible version of each bundled package is included with that Kibana version.
+Until further automation is added, this `fleet_packages.json` file should be updated as part of the release process to ensure the latest compatible version of each bundled package is included with that Kibana version. **This must be done before the final BC for a release is built.**
+Tracking issues should be opened and tracked by the Fleet UI team. See https://github.com/elastic/kibana/issues/129309 as an example.