docs: add node upgrade notes

docs/NODE_UPGRADE_NOTES.md

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+# Node Upgrade Notes
+
+This document summarizes a few best practices when upgrading a Base node.
+
+## 1. Back up data
+
+Before upgrading, back up:
+
+- the data directory (for example `geth-data`),
+- any local configuration files (`.env`, `config/*.env`),
+- Docker compose files if you have customized them.
+
+## 2. Check release notes
+
+Always check the release notes of the new version for:
+
+- breaking changes,
+- required flags or configuration changes,
+- migration steps.
+
+## 3. Rolling upgrade (Docker)
+
+For Docker-based setups:
+
+1. Stop the existing containers:
+
+       docker compose down
+
+2. Pull the new images:
+
+       docker pull <new-image-tags>
+
+3. Start the node again:
+
+       docker compose up -d
+
+4. Monitor logs for a few minutes to ensure the node syncs correctly.
+
+## 4. Rollback plan
+
+Keep a short rollback plan:
+
+- how to restore the previous image tag,
+- how to restore the backed up data directory.
+
+Having this written down reduces downtime if something goes wrong.