doc: add pep8 as the python code style guidelines

Added PEP8 to the DPDK Coding Style guidelines to cover Python
contributions to DPDK.

Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

doc/guides/contributing/coding_style.rst

@@ -685,3 +685,11 @@ Control Statements
                  usage();
                  /* NOTREACHED */
          }
+
+
+Python Code
+-----------
+
+All python code should be compliant with `PEP8 (Style Guide for Python Code) <https://www.python.org/dev/peps/pep-0008/>`_.
+
+The ``pep8`` tool can be used for testing compliance with the guidelines.