Added discussion about transforming COM to LAB system in documentation.
Added discussion about transforming COM to LAB system in documentation.

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