Add DEVELOPMENT.md with add-on technical notes

Author: andlo

DEVELOPMENT.md

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+# Development Notes
+
+This document captures technical findings and gotchas discovered while building this add-on.
+
+## Add-on structure
+
+- `config.yaml` — add-on metadata, options schema, port mappings
+- `Dockerfile` — builds on HA base image, installs Node.js, npm, jq and MeshCentral
+- `run.sh` — startup script using `bashio` for option reading and logging
+
+## MeshCentral config.json
+
+Valid options reference: https://config.meshcentraltools.com
+
+Known invalid/problematic options:
+- `cleanErrorLog` — **not a valid option**, do not use
+- `mstsc: false` — **disables RDP access**, omit entirely (default is true)
+
+### certUrl
+Required for agents outside the local network. Without it, agents cannot verify SSL and will fail to connect. The `run.sh` script updates this on every restart from the add-on `cert_url` option using `jq`.
+
+### Folder paths
+MeshCentral creates extra folders next to `--datapath` by default. We redirect them:
+- `--filespath` CLI flag → `DATA_PATH/meshcentral-files`
+- `settings.autoBackup.backupPath` → `DATA_PATH/meshcentral-backups`
+- `domains[""].sessionRecording.filepath` → `DATA_PATH/meshcentral-recordings`
+- `meshcentral-web` **cannot** be redirected (MeshCentral limitation)
+
+## GitHub Actions build
+
+- Uses `docker/build-push-action` for multi-arch builds (amd64, aarch64, armhf, armv7, i386)
+- Requires **Read and write permissions** under repo Settings → Actions → Workflow permissions
+- Images pushed to `ghcr.io/andlo/meshcentral-addon-{arch}`
+- Build triggers on push to `meshcentral/**` and on release publish
+
+## Related
+- [ha-meshcentral integration](https://github.com/andlo/ha-meshcentral) — HACS integration
+- [DEVELOPMENT.md in ha-meshcentral](https://github.com/andlo/ha-meshcentral/blob/main/DEVELOPMENT.md) — WebSocket API notes