Adding section to installation guide about configuring emacs nxml mode.
Adding section to installation guide about configuring emacs nxml mode.

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