Clarify language and targeted standard in README

I've noticed that some recent commits have introduced `nullptr` in the codebase and added a C++11 requirement to `CMakeLists.txt` (https://github.com/Interrupt/systemshock/commit/e07f23447f04c5726a07eab895e0d7f010aae71c). I think this is great and would welcome even more modern C++ standards, but regardless it needs to be documented in the README as contributors might not be aware that the project is not a C project anymore.