Updated release notes for 0.4.2.
Updated release notes for 0.4.2.

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