message: update superbuild README
  [ci skip]
diff: # OSPRay CMake Superbuild
  This CMake script will pull down OSPRay's dependencies and build OSPRay itself.
  -The result is an install directory with each dependency in its own directory.
  +The result is an install directory, with each dependency in its own directory.
  Run with:
  ```bash
  -git clone https://github.com/jeffamstutz/superbuild_ospray
  -cd superbuild_ospray
  mkdir build
  cd build
  -cmake ..
  -make
  +cmake [path/to/this/directory]
  +cmake --build .
  ```
  The resulting `install` directory will have everything in it.
  @@ -20,4 +18,7 @@ CMake options to note (all have sensible defaults):
  - `CMAKE_INSTALL_PREFIX` will be the root directory where everything gets installed.
  - `NUM_BUILD_JOBS` sets the number given to `make -j` for parallel builds.
  +- `BUILD_EMBREE_FROM_SOURCE` set to OFF will download a pre-built version of Embree.
  - `BUILD_EMBREE_VERSION` determines which verison of Embree to pull down.
  +- `BUILD_OIDN_FROM_SOURCE` set to OFF will download a pre-built version of OpenImageDenoise.
  +- `BUILD_OIDN_VERSION` determines which verison of OpenImageDenoise to pull down.
  