Add CONTRIBUTING.md: branch → PR → merge workflow

CONTRIBUTING.md

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+# Contributing to glogout
+
+Thanks for your interest in glogout! A few conventions keep the project tidy.
+
+## Branch → PR → merge
+
+**`main` is protected — no direct pushes.** All changes, including the
+maintainer's, go through a pull request:
+
+```bash
+git switch -c my-change      # branch off main
+# ...make your changes, commit them...
+git push -u origin my-change
+gh pr create                 # or open the PR in the GitHub UI
+```
+
+A PR can be merged into `main` once the **`build (release)`** CI check passes.
+This applies to solo work too — the PR is the review checkpoint and keeps the
+public history clean.
+
+## Building locally
+
+```bash
+cargo build --release
+```
+
+You'll need the system libraries listed under **Requirements** in the
+[README](README.md): GTK 4.14+, `gtk4-layer-shell`, and WebKitGTK 6.0.
+
+## Scope
+
+glogout targets Wayland compositors implementing `wlr-layer-shell` (validated on
+Hyprland). See the README for supported-compositor details and known
+limitations before filing a bug.