update docs for cmake
update docs for cmake

yjwong/s2-geometry-library,yjwong/s2-geometry-library,yjwong/s2-geometry-library,Noctem/pogeo,micolous/s2-geometry-library,micolous/s2-geometry-library,micolous/s2-geometry-library,laramiel/s2-geometry-library,Noctem/pogeo,Noctem/pogeo,yjwong/s2-geometry-library,Noctem/pogeo,laramiel/s2-geometry-library,micolous/s2-geometry-library,laramiel/s2-geometry-library