Add OPENMP Makefile variable to installation instructions.
Add OPENMP Makefile variable to installation instructions.

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