Update C++ style guide reference

[apazylbe]

The contributing page advises contributors to make sure that the code "follows the Google C++ Style Guide." We currently don't follow the Google style guide. We should probably remove the reference and document the style guide we want to follow, and update .clang-format if necessary

[hysw]

We should consider having a .clang-tidy so we can automatically check the naming convention. The pPointer, mMember and kConstant stuff still trips me over all the time.