docs: Add CONTRIBUTING.md with Branch/PR workflow

Author: Fleonex-dev

CONTRIBUTING.md

@@ -0,0 +1,36 @@
+# Contributing to System Design Masterclass
+
+## 🚀 The Golden Rule: Branch & Merge
+We follow a strict **Feature Branch Workflow** to ensure stability.
+The `main` branch should always be deployable (and passing tests).
+
+### 1. Create a Branch
+Never work on `main` directly.
+```bash
+git checkout -b feat/my-new-feature
+# or
+git checkout -b fix/concurrency-bug
+```
+
+### 2. Verify Locally
+Before pushing, ensure you haven't broken anything.
+```bash
+make test
+```
+
+### 3. Push & Open Pull Request
+```bash
+git push origin feat/my-new-feature
+```
+- Go to GitHub and open a **Pull Request (PR)**.
+- **Wait for CI**: Our GitHub Actions pipeline will automatically run.
+- **Green Check?** ✅ -> Safe to Merge.
+- **Red X?** ❌ -> Fix code, commit, and push again (PR updates automatically).
+
+### 4. Code Style
+- **Python**: We use `pylint` and `pytest`.
+- **Naming**: Snake_case for functions/files (`01_rate_limiter.py`).
+- **Simulations**: Scripts should demonstrate a concept and exit gracefully.
+
+---
+*Happy Coding!*