Mention flake8 and check-style.sh in CONTRIBUTING (pybind#1567)

CONTRIBUTING.md

@@ -27,6 +27,8 @@ adhere to the following rules to make the process as smooth as possible:
   do add value by themselves.
 * Add tests for any new functionality and run the test suite (``make pytest``)
   to ensure that no existing features break.
+* Please run ``flake8`` and ``tools/check-style.sh`` to check your code matches
+  the project style. (Note that ``check-style.sh`` requires ``gawk``.)
 * This project has a strong focus on providing general solutions using a
   minimal amount of code, thus small pull requests are greatly preferred.