Added instructions for modifying XML input in devguide.
Added instructions for modifying XML input in devguide.

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