Update CONTRIBUTING.md

To reflect the 'make format' new target.

Author: ChinYikMing

CONTRIBUTING.md

@@ -55,6 +55,7 @@ To maintain a uniform style across languages, run:
 * `clang-format -i *.{c,h}` to apply the project’s C/C++ formatting rules from the up-to-date .clang-format file.
 * `shfmt -w $(find . -type f -name "*.sh")` to clean and standardize all shell scripts.
 * `black .` to enforce a consistent, idiomatic layout for Python code.
+* `make format` to automatically run all three formatters.
 
 ## Coding Style for Shell Script