Update CONTRIBUTING.md (#1670)

Add missing C++ and Python code style links

Python code style link is taken from this old commit 32ba12c

Author: grif-on

CONTRIBUTING.md

@@ -47,8 +47,8 @@ $ git checkout dev
 
 The patch must be _complete_. And by that, we mean:
 
--   For C++ code follow the [axmol C++ Coding Style][1]
--   For Python code follow the [PEP8 guidelines][3]
+-   For C++ code follow the [axmol C++ Coding Style](https://github.com/axmolengine/axmol/blob/d8d3137515a73e5c2395cae04d6b5ef6b1cc6777/CONTRIBUTING.md)
+-   For Python code follow the [PEP8 guidelines](https://www.python.org/dev/peps/pep-0008)
 -   Describe what the patch does
 -   Include test cases if applicable
 -   Include unit tests if applicable