Add README for GitHub Actions build and release workflow detailing triggers, platforms, and artifact generation

.github/README.md

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+# GitHub Actions
+
+## Build and Release Workflow
+
+The `release.yml` workflow automatically builds the launcher for all platforms.
+
+### Triggers
+
+| Trigger | Builds | Creates Release |
+|---------|--------|-----------------|
+| Push to `main` | Yes | No |
+| Push tag `v*` | Yes | Yes |
+| Manual dispatch | Yes | No |
+
+### Platforms
+
+All builds run in parallel:
+
+- **Linux** (ubuntu-latest): AppImage, deb
+- **Windows** (windows-latest): NSIS installer, portable exe
+- **macOS** (macos-latest): Universal DMG (Intel + Apple Silicon)
+
+### Creating a Release
+
+1. Update version in `package.json`
+2. Commit and push to `main`
+3. Create and push a version tag:
+
+```bash
+git tag v2.0.1
+git push origin v2.0.1
+```
+
+The workflow will:
+1. Build all platforms in parallel
+2. Upload artifacts to GitHub Release
+3. Generate release notes automatically
+
+### Build Artifacts
+
+After each build, artifacts are available in the Actions tab for 90 days:
+
+- `linux-builds`: `.AppImage`, `.deb`
+- `windows-builds`: `.exe`
+- `macos-builds`: `.dmg`, `.zip`, `latest-mac.yml`
+
+### Local Development
+
+Build locally for your platform:
+
+```bash
+npm run build:linux
+npm run build:win
+npm run build:mac
+```
+
+Or build all platforms (requires appropriate OS):
+
+```bash
+npm run build:all
+```