minor: revise README.md to remove unneeded/duplicate sections
minor: revise README.md to remove unneeded/duplicate sections

acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver