Adding information about the DAGMC_ROOT variable.
Adding information about the DAGMC_ROOT variable.

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