Clarify changelog instructions (#232)

Author: povilasv

AGENTS.md

@@ -9,3 +9,11 @@ These instructions apply to all development via the Codex web UI.
   3. Run `make generate-examples CHARTS=opentelemetry-collector` to regenerate example manifests.
   4. Commit the regenerated examples along with your other changes.
 
+### Updating Changelog
+- Add new entry to `charts/opentelemetry-collector/CHANGELOG.md`.
+- Use current date in YYYY-MM-DD format (get with `date +%Y-%m-%d`).
+- Copy the entries from the opentelemetry-collector changelog.
+- Follow the existing format: `### vX.Y.Z / YYYY-MM-DD`.
+
+Make sure to update the version only once to avoid duplicate changelog entries.
+