add c++

Author: wing328

CONTRIBUTING.md

@@ -30,6 +30,7 @@ For a list of variables available in the template, please refer to this [page](h
 Code change should conform to the programming style guide of the respective langauages:
 - Android: https://source.android.com/source/code-style.html
 - C#: https://msdn.microsoft.com/en-us/library/vstudio/ff926074.aspx
+- C++: https://google.github.io/styleguide/cppguide.html
 - Haskell: https://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md
 - Java: https://google.github.io/styleguide/javaguide.html
 - JavaScript: https://github.com/airbnb/javascript/tree/master/es5