Add section Coding style

Author: ufechner7

CONTRIBUTING.md

@@ -31,4 +31,11 @@ It is OK to merge your own pull request without review if you are very sure it w
 - Fix division-by-zero error
 - Improve docstrings
 - Implement feature #23
-- Add function cool_feature()
+- Add function cool_feature()
+
+#### Coding style
+1. variable and function names lower case, use underscore if needed
+1. Types and structs CamelCase
+1. indent with four spaces
+1. max 120 char per line
+1. do NOT use an automated code formatter