update README to include variables for MPI

[rzambre]

Setting USE_MPI=1 alone does not configure Legion to use the MPI
backend. It must be coupled with REALM_NETWORKS=mpi.

[rzambre]

The README currently does not mention the existence of the REALM_NETWORKS variable. That documentation is hidden in the CHANGES.txt file.

[streichler]

I agree that the README should probably talk about MPI network layer, but that's not what USE_MPI=1 does. USE_MPI=1 enables the MPI compatibility logic in Legion and Realm. Is that what you're trying to get, or did you actually want the MPI
network layer in Realm?

[rzambre]

I see. I want to actually use the MPI backend in Realm.
Is it possible to use the MPI network layer with just REALM_NETWORKS=mpi and without USE_MPI=1 ?

[streichler]

Is it possible to use the MPI network layer with just REALM_NETWORKS=mpi and without USE_MPI=1 ?

Yes, REALM_NETWORKS=mpi should be sufficient on its own.

[rzambre]

Got it, I have updated the PR to reflect the availability of the REALM_NETWORKS variable.

[streichler]

Thanks. Rather than taking this little change as is, I'm inclined to use this PR as a forcing function to do a larger update of those docs to include all the other build switches we've been bad at documenting over the years. :)

[rzambre]

Sounds great :)