feat: add README for Navidrome to MusicBrainz ratings push script

README.md

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+# Navidrome to MusicBrainz Ratings Push
+
+This script reads ratings from Navidrome over its Subsonic API and submits them to MusicBrainz.
+
+It supports:
+
+- song ratings -> MusicBrainz recordings
+- album ratings -> MusicBrainz release groups
+- artist ratings -> MusicBrainz artists
+
+For songs, it will also expand a release-group match and rate every matching recording it can find in that group instead of stopping at the first one.
+
+## Setup
+
+1. Install dependencies:
+
+   `python -m pip install -r requirements.txt`
+
+Optional: run inside a Python virtual environment (recommended)
+
+- Windows (PowerShell):
+
+   ```powershell
+   python -m venv .venv
+   .\.venv\Scripts\Activate.ps1
+   python -m pip install -r requirements.txt
+   ```
+
+- Windows (cmd.exe):
+
+   ```cmd
+   python -m venv .venv
+   .\.venv\Scripts\activate.bat
+   python -m pip install -r requirements.txt
+   ```
+
+- Unix/macOS:
+
+   ```bash
+   python3 -m venv .venv
+   source .venv/bin/activate
+   python -m pip install -r requirements.txt
+   ```
+
+2. Set your Navidrome and MusicBrainz credentials:
+
+   - `NAVIDROME_BASE_URL`
+   - `NAVIDROME_USERNAME`
+   - `NAVIDROME_PASSWORD`
+
+   - `MB_USERNAME`
+   - `MB_PASSWORD`
+
+3. Run the script:
+
+   `python musicbrainz-ratings-helper.py`
+
+## Useful flags
+
+- `--navidrome-base-url`, `--navidrome-username`, and `--navidrome-password` to override environment variables
+- `--entity song --entity album --entity artist` to limit what gets exported
+- `--dry-run` to preview submissions
+- `--max-targets 1` to limit a dry run to the first resolved target
+- `--no-expand-release-groups` to disable release-group fan-out for songs
+
+## Notes
+
+- The script submits one through five star ratings and converts them to MusicBrainz's 0-100 scale.
+- Zero ratings are skipped.
+- MusicBrainz requires authenticated XML POST submissions and a meaningful client string.
+- Navidrome access uses the Subsonic API only; the script does not read the database directly.