Update Branson docs here to indicate no need for Metis (and how to disable it if necessary)

[gshipman]

@alexrlongne will be updating Branson Readme to indicate no need for Metis in certain cases, we should document this here as well.

[alexrlongne]

Documentation is now up to date regarding OpenMP and Metis in the develop branch of Branson at https://github.com/lanl/branson/

[gshipman]

Done!

[gshipman]

@gshipman needs to check this is in the docs on benchmark site.

[gshipman]

Closed! Docs updated to reflect single node without Metis, multi with Metis.