googletest

Added CMake configure-time download instructions to docs

Adds instructions for how to add gtest and gmock to
another CMake project directly. Downloading of the
googletest sources happens as configure time, allowing
it to be added to the main build directly via the
add_subdirectory() command. This ensures googletest is
built with the same compiler settings, etc. and will
typically result in a more robust and more convenient
build arrangement.