Add CONTRIBUTING.md with branching and release workflow

Author: Revisto

CONTRIBUTING.md

@@ -0,0 +1,41 @@
+# Contributing
+
+## Branches
+
+- `dev` — default branch, all PRs go here
+- `main` — releases only, string frozen for translations
+
+## Workflow
+
+1. Create a branch from `dev`
+2. Open a PR targeting `dev`
+3. Merge to `dev`
+
+## Releasing
+
+1. Create a release branch from `dev`:
+   ```bash
+   git checkout dev
+   git pull
+   git checkout -b release/X.Y.Z
+   ```
+2. Update version in `meson.build` following [Semantic Versioning](https://semver.org/)
+3. Update `data/io.github.revisto.drum-machine.metainfo.xml` with release notes
+4. Commit and push:
+   ```bash
+   git add meson.build data/io.github.revisto.drum-machine.metainfo.xml
+   git commit -m "Release X.Y.Z"
+   git push origin release/X.Y.Z
+   ```
+5. Open a PR targeting `main` and merge it
+6. Announce string freeze on [GNOME Discourse](https://discourse.gnome.org/) and Matrix room
+
+7. Wait for string freeze (1-2 weeks depending on changes)
+
+8. Tag and push after translations are complete:
+   ```bash
+   git tag vX.Y.Z
+   git push origin --tags
+   ```
+
+9. Release to Flathub