Documentation: adding OpenMC dependency

[hmarden]

I dug through the commits to find a version of the adapter that would work with OpenMC v0.15.0: later commits than 872e882 were trying to import the OrthogonalBox class which was introduced in v0.15.1.

Perhaps the OpenMC dependency could be added to the ReadMe or the pyproject.toml?

[MicahGale]

I think adding it to pyproject.toml would be a good idea. If you have it installed already (which you should) pip won't try to install openmc