readme: document embedded engines
readme: document embedded engines

cheunghy/slim,etdev/slim,yterajima/slim,slim-template/slim,XueQian/slim,JuanitoFatas/slim,slim-template/slim,zakuro9715/slim,ericboehs/slim,slim-template/slim,mojavelinux/slim,metallurgical/slim,zakuro9715/slim,ericboehs/slim,JuanitoFatas/slim,fly19890211/slim,etdev/slim,metallurgical/slim,Nikeshsuwal/slim,ruijsun/slim,sideci-sample/sideci-sample-slim,fly19890211/slim,Nikeshsuwal/slim,cheunghy/slim,ruijsun/slim,raxoft/slim,mojavelinux/slim,XueQian/slim,raxoft/slim