Document the release process in docs/RELEASING.md

docs/RELEASING.md

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+# Releasing a new version
+
+The PCM addon zip is built by CI (`.gitea/workflows/build-pcm.yml`).
+Every push to `main` builds it as a downloadable artifact; pushing a
+`v<version>` tag additionally creates a Gitea release with the zip
+attached. The release job checks that the tag matches `metadata.json`
+and fails on a mismatch.
+
+## Steps
+
+1. **Bump the version** in `metadata.json` — the single entry in
+   `versions` (plain `MAJOR.MINOR.PATCH`, no `v` prefix; the PCM schema
+   rejects anything else):
+
+   ```json
+   "versions": [
+       {
+           "version": "1.0.2",
+           "status": "stable",
+           "kicad_version": "10.0",
+           "runtime": "ipc"
+       }
+   ]
+   ```
+
+2. **Commit, tag, push** (tag = `v` + the manifest version):
+
+   ```powershell
+   git add metadata.json
+   git commit -m "Release 1.0.2"
+   git tag v1.0.2
+   git push
+   git push origin v1.0.2
+   ```
+
+3. **Verify**: the Actions run for the tag builds
+   `th.co.b4l.fill-resistance_<version>.zip` and publishes it at
+   <https://git.b4l.co.th/B4L/kicad-zone-resistance/releases>, together
+   with `metadata-registry.json`. The zip installs directly via
+   Plugin and Content Manager → *Install from File*.
+
+## Publishing to the official KiCad registry (optional)
+
+The attached `metadata-registry.json` already carries the release
+`download_url`, `download_sha256` and sizes. Submit it as
+`packages/th.co.b4l.fill-resistance/metadata.json` in a merge request
+to <https://gitlab.com/kicad/addons/metadata>. The registry keeps every
+published version: append the new entry to the `versions` array of the
+registry copy instead of replacing the previous one (the repo's own
+`metadata.json` only ever holds the current version —
+`tools/build_package.py` reads `versions[0]`).
+
+## Local build (no CI)
+
+```powershell
+python tools/build_package.py    # writes dist/<identifier>_<version>.zip
+```
+
+Pure stdlib — no venv needed. `dist/` is gitignored.
+
+## CI prerequisites (one-time, server side)
+
+- Actions enabled for the repo (Settings → Actions unit).
+- A runner registered with the `ubuntu-latest` label; the default
+  act_runner image works — the build needs only Python 3.
+- Workflow actions are pinned to commit SHAs; when bumping them, update
+  the SHA and the trailing version comment together.