message: Add link to architecture selector to README
diff: @@ -130,7 +130,10 @@ Ginkgo adds the following additional switches to control what is being built:
  all architectures of that particular NVIDIA GPU generation. `COMPUTE` and
  `CODE` are placeholders that should be replaced with compute and code
  numbers (e.g. for `compute_70` and `sm_70` `COMPUTE` and `CODE` should be
  - replaced with `70`. Default is `Auto`.
  + replaced with `70`. Default is `Auto`. For a more detailed explanation of
  + this option see the
  + [`ARCHITECTURES` specification list](https://github.com/ginkgo-project/CudaArchitectureSelector/blob/master/CudaArchitectureSelector.cmake#L58)
  + section in the documentation of the CudaArchitectureSelector CMake module.
  For example, to build everything (in debug mode), use:
  