Update style guide with rule on whitespace.
Update style guide with rule on whitespace.

liangjg/openmc,wbinventor/openmc,johnnyliu27/openmc,smharper/openmc,johnnyliu27/openmc,shikhar413/openmc,smharper/openmc,walshjon/openmc,wbinventor/openmc,walshjon/openmc,paulromano/openmc,walshjon/openmc,mit-crpg/openmc,bhermanmit/openmc,amandalund/openmc,paulromano/openmc,wbinventor/openmc,amandalund/openmc,paulromano/openmc,smharper/openmc,wbinventor/openmc,smharper/openmc,mit-crpg/openmc,johnnyliu27/openmc,mit-crpg/openmc,walshjon/openmc,shikhar413/openmc,shikhar413/openmc,amandalund/openmc,shikhar413/openmc,liangjg/openmc,liangjg/openmc,bhermanmit/openmc,amandalund/openmc,johnnyliu27/openmc,paulromano/openmc,mit-crpg/openmc,liangjg/openmc