Add prerequisites to getting started doc.
Add prerequisites to getting started doc.

Change-Id: Id5e31752f30095660ddff20367e821d3e344c692

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