Add description of serialized XML file to README
Add description of serialized XML file to README

mit-crpg/openmc,liangjg/openmc,paulromano/openmc,shikhar413/openmc,mit-crpg/openmc,walshjon/openmc,amandalund/openmc,amandalund/openmc,shikhar413/openmc,shikhar413/openmc,amandalund/openmc,amandalund/openmc,paulromano/openmc,liangjg/openmc,mit-crpg/openmc,liangjg/openmc,walshjon/openmc,walshjon/openmc,shikhar413/openmc,paulromano/openmc,liangjg/openmc,walshjon/openmc,mit-crpg/openmc,paulromano/openmc