Clarify multithreading requirements

Author: giordano

exercises/multithreading/README.md

@@ -1,5 +1,7 @@
 ## Requirements
 
+Only needed if you're running this not on NERSC or GCP:
+
 ```
 julia --project=. -e 'using Pkg; Pkg.instantiate(); using IJulia; IJulia.installkernel("Multithreaded Julia", "--project=@.", "--threads=auto")'
 ```