Add specific HPC install recommendation (#3161)

* Add specific HPC install instructions

* Update README.md

Author: jhale

README.md

@@ -73,6 +73,8 @@ comprehensive instructions.
 - Windows: [docker](#docker-images), or install
   [WSL2](https://docs.microsoft.com/en-us/windows/wsl/install) and use
   [Ubuntu](#ubuntu-packages).
+- High performance computers: [Spack](#spack) or
+  [from source](#from-source), both using system-provided MPI.
 
 #### conda