added usage instrucions
added usage instrucions

shikhar413/openmc,amandalund/openmc,paulromano/openmc,paulromano/openmc,shikhar413/openmc,amandalund/openmc,amandalund/openmc,walshjon/openmc,walshjon/openmc,walshjon/openmc,walshjon/openmc,paulromano/openmc,shikhar413/openmc,amandalund/openmc,shikhar413/openmc,paulromano/openmc