added documentation on working with FoX submodule
added documentation on working with FoX submodule

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