Add use of Flake8 to contributing.txt

contributing.txt

@@ -19,6 +19,14 @@ following:
 - all new features must come with a test (or several if it is
   a big feature)
 
+Meson uses Flake8 for style guide enforcement. The Flake8 options for
+the project are contained in setup.cfg.
+
+To run Flake8 on your local clone of Meson:
+
+ $ python3 -m pip install flake8
+ $ cd meson
+ $ flake8
 
 C/C++ coding style