Update C++ Readme

[mknaranja]

Provide a description of requested documentation changes.

The C++ Readme references third party dependency versions of MEmilio and mentions available Build options (e.g. we miss MEMILIO_ENABLE_MPI). This page is slightly out of date and should be updated, among other things, via the information in:

• https://github.com/DLR-SC/memilio/blob/main/.github/actions/linux-build/action.yml
• https://github.com/DLR-SC/memilio/blob/main/cpp/CMakeLists.txt