Added short section on DBRC in documentation. Closes #433.
Added short section on DBRC in documentation. Closes #433.

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