README: link to the design doc
README: link to the design doc

Change-Id: Ibb649ad7b1fbbbbf1dc4ad604982e1502f957ee3

project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification,project-oak/hafnium-verification