docs: add branching strategy

docs/branching-strategy.md

@@ -0,0 +1,15 @@
+# Branching-Strategie
+
+```
+main
+└── develop
+    ├── feature/<name>
+    ├── fix/<name>
+    └── chore/<name>
+```
+
+- `main` – stabiler Produktionsstand, nur Merges aus `develop`
+- `develop` – Integrationsstand, Basis für alle Feature-Branches
+- `feature/*` – neue Features, Basis: `develop`
+- `fix/*` – Bugfixes, Basis: `develop`
+- `chore/*` – Wartung, Konfiguration, Basis: `develop`