Document how private overlay changes are synced

[georgeolaru]

Summary

The repo documents how to bootstrap the private overlay, but it does not explain how new or changed private files are handled after the first clone.

Problem

It is not obvious when bin/bootstrap-private picks up changes automatically, when --force is needed, and when the script itself must be extended for a new private path.

Goal

Document the ongoing maintenance flow for private overlay files in the public repo instructions.