Update instructions for MPICH2/OpenMPI.
Update instructions for MPICH2/OpenMPI.

lilulu/openmc,lilulu/openmc,keadyk/openmc_mg_prepush,mjlong/openmc,walshjon/openmc,smharper/openmc,walshjon/openmc,amandalund/openmc,paulromano/openmc,paulromano/openmc,mit-crpg/openmc,keadyk/openmc_mg_prepush,shikhar413/openmc,samuelshaner/openmc,mit-crpg/openmc,mjlong/openmc,shikhar413/openmc,shikhar413/openmc,kellyrowland/openmc,shenqicang/openmc,kellyrowland/openmc,walshjon/openmc,shikhar413/openmc,johnnyliu27/openmc,liangjg/openmc,liangjg/openmc,walshjon/openmc,smharper/openmc,keadyk/openmc_mg_prepush,mit-crpg/openmc,shenqicang/openmc,wbinventor/openmc,johnnyliu27/openmc,nhorelik/openmc,johnnyliu27/openmc,samuelshaner/openmc,smharper/openmc,samuelshaner/openmc,bhermanmit/openmc,johnnyliu27/openmc,nhorelik/openmc,amandalund/openmc,amandalund/openmc,bhermanmit/cdash,sxds/opemmc,paulromano/openmc,samuelshaner/openmc,mit-crpg/openmc,lilulu/openmc,wbinventor/openmc,liangjg/openmc,smharper/openmc,wbinventor/openmc,wbinventor/openmc,amandalund/openmc,bhermanmit/openmc,paulromano/openmc,liangjg/openmc,sxds/opemmc