about.txt

This was the real use case benmet was used in.
The code uses `benmet.step_templates`, which I didn't end up liking.
I think implementing the same logic in the step scripts themselves is probably more readable, even if you end up repeating yourself, but it is what it is.

The steps require two repositories (add them using benmet's `add-repos` command):
 - Open MPI: https://github.com/open-mpi/ompi.git
 - A private development version of ReproMPI. The public version is https://github.com/hunsa/reprompi.git , however you'll have to adjust the parameters to not use the ones that aren't implemented there.

I also frankly don't know what libraries and further setup you need to build Open MPI. I just know that this build procedure already worked on the server that I ran it on.