[Doc] Update contribution guidelines for better onboarding (vllm-proj…

…ect#3819)

CONTRIBUTING.md

@@ -30,6 +30,8 @@ pip install -e .  # This may take several minutes.
 ```bash
 pip install -r requirements-dev.txt
 
+# linting and formatting
+bash format.sh
 # Static type checking
 mypy
 # Unit tests