Mention mpi4py as optional dependency in docs
Mention mpi4py as optional dependency in docs

shikhar413/openmc,johnnyliu27/openmc,johnnyliu27/openmc,amandalund/openmc,amandalund/openmc,shikhar413/openmc,walshjon/openmc,mit-crpg/openmc,paulromano/openmc,wbinventor/openmc,johnnyliu27/openmc,amandalund/openmc,mit-crpg/openmc,walshjon/openmc,liangjg/openmc,wbinventor/openmc,mit-crpg/openmc,johnnyliu27/openmc,shikhar413/openmc,smharper/openmc,paulromano/openmc,wbinventor/openmc,liangjg/openmc,walshjon/openmc,smharper/openmc,walshjon/openmc,amandalund/openmc,paulromano/openmc,shikhar413/openmc,wbinventor/openmc,liangjg/openmc,liangjg/openmc,mit-crpg/openmc,smharper/openmc,paulromano/openmc,smharper/openmc