Added documentation.
Added documentation.

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