new supplementary page on forking

Author: DininduSenanayake

docs/supplementary/2.forking.md

@@ -0,0 +1,30 @@
+# Forking — External Contributions
+
+A postdoc from another university wants to adapt `CellClusterFlow` for their spatial transcriptomics data. They can't push directly (no write access), so they **fork** the repository.
+
+### Forking workflow:
+
+1. **Fork**: Click "Fork" on GitHub → creates copy under their account
+2. **Clone their fork**:
+````bash
+   git clone https://github.com/PostdocZ/CellClusterFlow.git
+````
+3. **Make changes** on a new branch
+4. **Push to their fork**
+5. **Open a Pull Request** from their fork back to Dr. X's original repo
+````mermaid
+flowchart TB
+  ORIG[DrX/CellClusterFlow<br/>Original repo] 
+  FORK[PostdocZ/CellClusterFlow<br/>Forked repo]
+  
+  ORIG -->|Fork| FORK
+  FORK -->|Make changes| FORK
+  FORK -->|Pull Request| ORIG
+  
+  style ORIG fill:#e6ffe6,stroke:#333
+  style FORK fill:#e6f7ff,stroke:#333
+````
+
+This is how **open-source** contributions work — anyone can suggest improvements without needing direct access!
+
+---