Mention command-line flags in user's guide.
Mention command-line flags in user's guide.

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