Add prereq and build info to readme
Add prereq and build info to readme

cesarghali/keytransparency,gdbelvin/keytransparency,AMarcedone/keytransparency,cesarghali/keytransparency,google/keytransparency,alsophian/keytransparency,AMarcedone/keytransparency,google/keytransparency,gdbelvin/keytransparency,alsophian/keytransparency