message: Update HWLOC instructions as well.
diff: @@ -249,19 +249,21 @@ packages can be turned off by disabling the relevant options.
  [git-cmake-format](https://github.com/gflegar/git-cmake-format) is our CMake
  helper for code formatting.
  + GINKGO_BUILD_HWLOC=ON:
  - [hwloc](https://www.open-mpi.org/projects/hwloc) to help us to control cores
  + [hwloc](https://www.open-mpi.org/projects/hwloc) to detect and control cores
  and devices.
  Usually, Ginkgo uses the internal version of each package. For each of the
  packages `GTEST`, `GFLAGS`, `RAPIDJSON` and `CAS`, it is possible to force
  Ginkgo to try to use an external version of a package. For this, Ginkgo provides
  two ways to find packages. To rely on the CMake `find_package` command, use the
  -CMake option `-DGINKGO_USE_EXTERNAL_<package>=ON`. `HWLOC` always looks for
  -system `HWLOC` first currently. Note that, if the external packages were not
  -installed to the default location, the CMake option
  -`-DCMAKE_PREFIX_PATH=<path-list>` needs to be set to the semicolon (`;`)
  -separated list of install paths of these external packages. For more
  -Information, see the [CMake documentation for
  +CMake option `-DGINKGO_USE_EXTERNAL_<package>=ON`. `HWLOC` works the opposite
  +way, Ginkgo always looks for the system's `hwloc` first. Set
  +`-DGINKGO_USE_EXTERNAL_HWLOC=OFF` to not use the system provided one.
  +
  +Note that, if the external packages were not installed to the default location,
  +the CMake option `-DCMAKE_PREFIX_PATH=<path-list>` needs to be set to the
  +semicolon (`;`) separated list of install paths of these external packages. For
  +more Information, see the [CMake documentation for
  CMAKE_PREFIX_PATH](https://cmake.org/cmake/help/v3.9/variable/CMAKE_PREFIX_PATH.html)
  for details.
  